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{{Short description|Warm-blooded animals with wings and feathers}}
{{Redirect-several|Bird|Birds|Aves|Avian|Avifauna}}
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{{Use British English|date=September 2016}}
{{Use dmy dates|date=August 2024}}
{{Automatic taxobox
| name = Birds
| fossil_range = <br />{{fossilrange|72|0|[[Maastrichtian]] – [[Holocene|present]], 72–0 [[Year#SI prefix multipliers|Ma]]| | earliest=115.5|PS= |ref=<ref name="Field2020">{{Cite journal|title=Late Cretaceous neornithine from Europe illuminates the origins of crown birds|last1=Field|first1=Daniel J.|last2=Benito|first2=Juan|date=March 2020|journal=Nature|issue=7799|doi=10.1038/s41586-020-2096-0|volume=579|pages=397–401 |last3=Chen|first3=Albert|last4=Jagt|first4=John W. M.|last5=Ksepka|first5=Daniel T.|pmid=32188952|bibcode=2020Natur.579..397F |url=https://www.repository.cam.ac.uk/handle/1810/303639|url-access=subscription}}</ref><ref name="DePietriWilaru">{{cite journal |last1=De Pietri |first1=Vanesa L. |last2=Scofield |first2=R. Paul |last3=Zelenkov |first3=Nikita |last4=Boles |first4=Walter E. |last5=Worthy |first5=Trevor H. |title=The unexpected survival of an ancient lineage of anseriform birds into the Neogene of Australia: the youngest record of Presbyornithidae |journal=Royal Society Open Science |date=February 2016 |volume=3 |issue=2 |article-number=150635 |doi=10.1098/rsos.150635 |pmid=26998335 |pmc=4785986 |bibcode=2016RSOS....350635D |doi-access=free }}</ref>}}
| image = <div class="center"><imagemap>
File:Bird Diversity 2013.png|300px
rect 0 0 333 232 [[Red-crested turaco]]
rect 0 232 333 470 [[Steller's sea eagle]]
rect 0 696 333 470 [[Feral pigeon]]
rect 0 928 333 700 [[Southern cassowary]]
rect 0 1160 333 930 [[Gentoo penguin]]
rect 0 1392 333 1160 [[Bar-throated minla]]

rect 666 0 333 232 [[shoebill]]
rect 666 232 333 470 [[grey crowned crane]]
rect 666 696 333 470 [[Anna's hummingbird]]
rect 666 928 333 700 [[rainbow lorikeet]]
rect 666 1160 333 930 [[grey heron]]
rect 666 1392 333 1160 [[Eurasian eagle-owl]]

rect 999 0 666 232 [[white-tailed tropicbird]]
rect 999 232 666 470 [[Indian peafowl]]
rect 999 696 666 470 [[Atlantic puffin]]
rect 999 928 666 700 [[American flamingo]]
rect 999 1160 666 930 [[blue-footed booby]]
rect 999 1392 666 1160 [[keel-billed toucan]]
</imagemap></div>
| display_parents = 8
| taxon = Aves
| authority = [[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]<ref>{{cite web| url= http://taxonomicon.taxonomy.nl/TaxonTree.aspx?id=80129&tree=0.1| title=Systema Naturae 2000 / Classification, Class Aves | access-date=11 June 2012 | last=Brands | first=Sheila | date=14 August 2008 | work=Project: The Taxonomicon }}</ref>
| subdivision_ranks = [[Neontology|Extant]] [[clade]]s
| subdivision = *[[Palaeognathae]] (ratites and tinamous)
** [[Struthioniformes]] (ostrich)
** [[Notopalaeognathae]]
* [[Neognathae]]
** [[Pangalloanserae]] (fowl)
** [[Neoaves]]
| synonyms = Neornithes <small>[[Hans Friedrich Gadow|Gadow]], 1883</small>
}}

<!--Please do not add any cites to the lead. The lead is meant to summarise the body which is already cited.-->
'''Birds''' are a group of [[warm-blooded]] [[vertebrate]] [[animal]]s constituting the [[class (biology)|class]] '''Aves''', characterised by [[feather]]s, toothless [[Beak|beaked]] jaws, the [[Oviparity|laying]] of [[Eggshell|hard-shelled]] eggs, a high [[Metabolism|metabolic]] rate, a four-chambered [[heart]], and a strong yet lightweight [[Bird skeleton|skeleton]]. Birds live worldwide and range in size from the {{cvt|5.5|cm}} [[bee hummingbird]] to the {{cvt|2.8|m}} [[common ostrich]]. There are over 11,000 living species and they are split into 44 [[Order (biology)|orders]]. More than half are [[passerine]] or "perching" birds. Birds have [[Bird wing|wings]] whose development varies according to species; the only known groups without wings are the extinct [[moa]] and [[elephant bird]]s. Wings, which are modified [[forelimb]]s, gave birds the ability to fly, although further evolution has led to the [[Flightless bird|loss of flight in some birds]], including [[ratite]]s, [[penguin]]s, and diverse [[endemism|endemic]] island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly [[seabird]]s and some [[Water bird|waterbirds]], have further evolved for swimming. The study of birds is called [[ornithology]].

Birds [[Evolution of birds|evolved]] from earlier [[theropods]], and thus constitute the only known living [[dinosaurs]]. Whilst traditional Linnaean taxonomy placed birds in their own class (owing to their unique morphology and being the only group of animals other than mammals where all its species are warm-blooded), birds are considered [[reptile]]s in the modern [[Cladistics|cladistic]] sense of the term. Their closest living relatives are the [[crocodilia]]ns, both forming a clade known as [[archosaurs]]. Birds are descendants of the primitive [[Avialae|avialans]] (whose members include ''[[Archaeopteryx]]'') which first appeared during the [[Late Jurassic]]. According to some estimates, modern birds ('''Neornithes''') evolved in the [[Late Cretaceous]] or between the Early and Late Cretaceous (100 Ma) and diversified dramatically around the time of the [[Cretaceous–Paleogene extinction event]] 66 million years ago, which killed off the [[pterosaur]]s and all non-[[ornithuran]] dinosaurs.<ref name=Claramunt2015/><ref name="Brocklehurst&Field2024">{{Cite journal |last1=Brocklehurst |first1=Neil |last2=Field |first2=Daniel J. |date=2024 |title=Tip dating and Bayes factors provide insight into the divergences of crown bird clades across the end-Cretaceous mass extinction |journal=Proceedings. Biological Sciences |volume=291 |issue=2016 |article-number=20232618 |doi=10.1098/rspb.2023.2618 |pmc=10865003 |pmid=38351798}}</ref>

Many [[social animal|social species]] preserve knowledge across generations ([[Animal culture#Examples of culturally transmitted behaviors in birds|culture]]). Birds are social, communicating with visual signals, calls, and [[bird vocalization|songs]], and participating in such behaviour as [[helpers at the nest|cooperative breeding]] and hunting, [[Flocking (behavior)|flocking]], and [[Mobbing (animal behavior)|mobbing]] of predators. The vast majority of bird species are socially (but not necessarily sexually) [[Monogamy in animals|monogamous]], usually for one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are [[Polygyny in animals|polygynous]] (one male with many females) or, rarely, [[Polyandry in nature|polyandrous]] (one female with many males). Birds produce offspring by laying eggs which are fertilised through [[sexual reproduction]]. They are usually laid in a nest and [[Avian incubation|incubated]] by the parents. Most birds have an extended period of parental care after hatching.

Many species of birds are economically important as food for human consumption and raw material in manufacturing, with [[Poultry|domesticated]] and [[Game (hunting)|undomesticated]] birds being important sources of eggs, meat, and feathers. [[Songbird]]s, parrots, and other species are popular as pets. [[Guano]] (bird excrement) is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become [[Bird extinction|extinct]] due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational [[birdwatching]] is an important part of the [[ecotourism]] industry.

==Evolution and classification==
{{Main|Evolution of birds}}

[[File:Archaeopteryx_lithographica_(Berlin_specimen).jpg|alt= Slab of stone with fossil bones and feather impressions|thumb|''[[Archaeopteryx]]'' is often considered the oldest known true bird.]]
The first [[biological classification|classification]] of birds was developed by [[Francis Willughby]] and [[John Ray]] in their 1676 volume ''Ornithologiae''.<ref>{{Cite book|last=del Hoyo |first=Josep |author2=Andy Elliott|author3=Jordi Sargatal |title=Handbook of Birds of the World, Volume 1: Ostrich to Ducks |year=1992 |publisher=[[Lynx Edicions]] |location=Barcelona |isbn=84-87334-10-5}}</ref>
[[Carl Linnaeus]] modified that work in 1758 to devise the [[taxonomic classification]] system currently in use.<ref>
{{Cite book|last=Linnaeus |first=Carolus |author-link=Carl Linnaeus |title=Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata |publisher=Holmiae. (Laurentii Salvii) |year=1758 |page=824 |language=la|title-link=Systema Naturae }}</ref> Birds are usually categorised as a [[Class (biology)|biological class]] in traditional [[evolutionary taxonomy]]. Modern [[phylogenetic taxonomy]] places Aves in the [[clade]] [[Theropoda]], sometimes with a Linnean rank lower than class, such as subclass<ref>{{Cite journal |last1=Ruggiero |first1=Michael A. |last2=Gordon |first2=Dennis P. |last3=Orrell |first3=Thomas M. |last4=Bailly |first4=Nicolas |last5=Bourgoin |first5=Thierry |last6=Brusca |first6=Richard C. |last7=Cavalier-Smith |first7=Thomas |last8=Guiry |first8=Michael D. |last9=Kirk |first9=Paul M. |date=2015-06-11 |title=Correction: A Higher Level Classification of All Living Organisms |journal=PLOS ONE |language=en |volume=10 |issue=6 |article-number=e0130114 |doi=10.1371/journal.pone.0130114 |doi-access=free |pmc=5159126 |pmid=26068874|bibcode=2015PLoSO..1030114R }}</ref> or infraclass.<ref name="Theropoda">{{Cite journal |date=January 2007 | title=Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion | journal=[[Zoological Journal of the Linnean Society]] | volume=149 | issue=1 | pages=1–95 | doi=10.1111/j.1096-3642.2006.00293.x | pmid=18784798 | last1=Livezey | first1=Bradley C. | last2=Zusi | first2=RL | pmc=2517308}}</ref>

===Definition===
Aves and a sister group, the order [[Crocodilia]], contain the only living representatives of the reptile clade [[Archosauria]]. During the late 1990s, Aves was most commonly defined [[Phylogenetics|phylogenetically]] as all descendants of the [[most recent common ancestor]] of modern birds and ''[[Archaeopteryx|Archaeopteryx lithographica]]''.<ref>{{Cite book|last=Padian|first=Kevin|author-link=Kevin Padian|author2=[[Philip J. Currie]]|title=Encyclopedia of Dinosaurs|year=1997|publisher=[[Academic Press]]|location=San Diego|pages=41–96|chapter=Bird Origins|isbn=0-12-226810-5}}</ref> However, an earlier definition proposed by [[Jacques Gauthier]] gained wide currency in the 21st century, and is used by many scientists including adherents to the ''[[PhyloCode]]''. Gauthier defined Aves to include only the [[crown group]] of the set of modern birds. This was done by excluding most groups known only from [[fossils]], and assigning them, instead, to the broader group Avialae,<ref name =Gauthier1986 >{{Cite book |last=Gauthier |first=Jacques |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor=Padian, Kevin |title=The Origin of Birds and the Evolution of Flight |series=Memoirs of the California Academy of Science |volume=8 |pages=1–55 |isbn=0-940228-14-9 |publisher=Published by California Academy of Sciences |location=San Francisco, CA|url=https://biostor.org/reference/110202}}</ref> on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.<ref name =Gauthier1986 />

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.<ref name="gauthier&dequeiroz2001"/> The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants,<ref name="gauthier&dequeiroz2001"/> which corresponds to meaning number 4 below. They assigned other names to the other groups.<ref name="gauthier&dequeiroz2001"/>

{{Cladogram
 |caption=The birds' phylogenetic relationships to major living reptile groups. {{small|(The [[turtle]]s' position is uncertain: Some authorities embed them inside the [[Archosaur]]s, with '''birds''' and [[crocodile]]s.)}}
 |clades={{clade
  |style=font-size:75%
  |label1=[[Reptile]]s
  |1= {{clade
     |label1=
     |1= {{clade
        |label1=[[Squamate]]s
        |1=  [[Lizard]]s & [[snake]]s
     }}
    |2= {{clade
      |label1=[[Pantestudines]]
      |1=  [[Turtle]]s
      |label2=[[Archosaur]]s
      |2= {{clade
          |label1=
          |1=  [[Crocodile]]s
          |2=  '''Birds'''
       }}
     }}
   }}
 }}
}}

# Aves can mean all [[archosaur]]s closer to birds than to [[crocodile]]s (alternately [[Avemetatarsalia]])
# Aves can mean those advanced archosaurs with feathers (alternately [[Avifilopluma]])
# Aves can mean those feathered dinosaurs that fly (alternately [[Avialae]])
# Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "[[crown group]]", in this sense synonymous with '''Neornithes''')

Under the fourth definition ''Archaeopteryx'', traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and [[bird evolution]], though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.<ref name=Nature>{{Cite journal|author1=Godefroit, Pascal |author2=Andrea Cau |author3=Hu Dong-Yu |author4=François Escuillié |author5=Wu Wenhao |author6=Gareth Dyke |year=2013 |title=A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds |journal=Nature |volume= 498|issue= 7454|pages= 359–362|doi=10.1038/nature12168 |pmid=23719374|bibcode=2013Natur.498..359G |author1-link=Pascal Godefroit }}</ref>

{{Cladogram|caption=Cladogram showing the results of a phylogenetic study by Cau, 2018.<ref name="paleoitalia"/>|clades=
{{clade
   | style=font-size:75%;line-height:80%
   |label1=Maniraptoromorpha
   |1={{clade
      |label1 =
      |1={{clade
         |label1 =
         |1= {{extinct}}''[[Coelurus]]''
         }}
      |2={{clade
         |1= {{extinct}}''[[Ornitholestes]]''
         |label2=[[Maniraptoriformes]]
         |2={{clade
            |1= {{extinct}}[[Ornithomimosauria]]
            |label2=[[Maniraptora]]
            |2={{clade
               |1= {{extinct}}[[Alvarezsauridae]]
               |label2=[[Pennaraptora]]
               |2={{clade
                  |label1 =
                  |1= {{extinct}}[[Oviraptorosauria]]
                  |2=  [[Paraves]]
                  }}
               }}
            }}
         }}
      }}
   }}
}}

Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all [[theropod]]s closer to birds than to ''[[Deinonychus]]''",<ref name="weishampel2004">{{cite book |editor-last1=Weishampel |editor-first1=David B. |editor-last2=Dodson |editor-first2=Peter |editor-last3=Osmólska |editor-first3=Halszka |year=2004 |title=The Dinosauria |edition=Second |publisher=University of California Press |pages=861 pp}}</ref><ref name="senter2007">{{cite journal |last1=Senter |first1=P. |year=2007 |title=A new look at the phylogeny of Coelurosauria (Dinosauria: Theropoda) |journal=Journal of Systematic Palaeontology |volume=5| issue=4| pages=429–463| doi=10.1017/S1477201907002143 |bibcode=2007JSPal...5..429S }}</ref> with ''[[Troodon]]'' being sometimes added as a second external specifier in case it is closer to birds than to ''Deinonychus''.<ref>{{Cite journal |last1=Maryańska|first1=Teresa|last2=Osmólska|first2=Halszka|last3=Wolsan|first3=Mieczysław |date=2002|title=Avialan status for Oviraptorosauria|journal=Acta Palaeontologica Polonica}}</ref> Avialae is also occasionally defined as an [[Phylogenetic nomenclature#Phylogenetic definitions|apomorphy-based clade]] (that is, one based on physical characteristics). [[Jacques Gauthier]], who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping [[Bird flight|flight]], and the birds that descended from them.<ref name="gauthier&dequeiroz2001">{{cite book |last1=Gauthier |first1=J. |last2=de Queiroz |first2=K. |year=2001 |chapter=Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves |pages=7–41 |title=New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom |editor-first1=J. A. |editor-last1=Gauthier |editor-first2=L. F. |editor-last2=Gall |publisher=Peabody Museum of Natural History, Yale University |location=New Haven, CT}}</ref><ref name="gauthier1986">{{cite book |last1=Gauthier |first1=J. |year=1986 |chapter=Saurischian monophyly and the origin of birds |editor-first1=K. |editor-last1=Padian |title=The origin of birds and the evolution of flight |location=San Francisco, California |publisher=Mem. Calif. Acad. Sci. |pages=1–55}}</ref>

Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma.<ref name=":1">{{cite book |doi=10.1016/B978-012676460-4/50004-4 |chapter=Crown-Clades, Key Characters and Taxonomic Stability: When is an Amniote Not an Amniote? |title=Amniote Origins |date=1997 |last1=Lee |first1=Michael S.Y. |last2=Spencer |first2=Patrick S. |pages=61–84 |isbn=978-0-12-676460-4 }}</ref>

===Dinosaurs and the origin of birds===
{{Main|Origin of birds}}

{{Cladogram|caption=Cladogram following the results of a phylogenetic study by Cau ''et al.'', 2015<ref name=cauetal2015>{{cite journal|doi=10.7717/peerj.1032|pmid=26157616|pmc=4476167|title=The phylogenetic affinities of the bizarre Late Cretaceous Romanian theropod ''Balaur'' bondoc(Dinosauria, Maniraptora): Dromaeosaurid or flightless bird?|journal=PeerJ|volume=3|article-number=e1032|year=2015|last1=Cau|first1=Andrea|last2=Brougham|first2=Tom|last3=Naish |first3=Darren |bibcode=2015PeerJ...3e1032C |doi-access=free }}</ref> |clades=
{{clade| style=font-size:75%;line-height:80%
|label1=[[Paraves]]
|1={{clade
  |1={{extinct}}[[Scansoriopterygidae]]
  |2={{clade
    |1={{extinct}}''[[Eosinopteryx]]''
    |label2=[[Eumaniraptora]]
    |2={{clade
     |1=†''[[Jinfengopteryx]]''
     |2=†''[[Aurornis]]''
     |3=†[[Dromaeosauridae]]
     |4=†[[Troodontidae]]
     |5=[[Avialae]] }} }} }} }} }}
[[File:Birds and dinosaurs.webp|left|thumb|upright=1.35|Simplified [[phylogenetic tree]] showing the relationship between modern birds and other dinosaurs<ref>{{Cite journal|last1=Plotnick|first1=Roy E.|last2=Theodor|first2=Jessica M. |last3=Holtz|first3=Thomas R.|date=24 September 2015|title=Jurassic Pork: What Could a Jewish Time Traveler Eat?|journal=Evolution: Education and Outreach|volume=8|issue=1|page=17 |doi=10.1186/s12052-015-0047-2 |doi-access=free|hdl=1903/27622|hdl-access=free}}</ref>]]

Based on [[fossil]] and biological evidence, most scientists accept that birds are a specialised subgroup of [[theropod]] [[dinosaur]]s<ref>{{Cite journal|last=Prum|first=Richard O. |title=Who's Your Daddy?|journal=Science|volume=322|pages=1799–1800|date=19 December 2008|doi=10.1126/science.1168808|pmid=19095929|issue=5909 |doi-access=free}}</ref> and, more specifically, members of [[Maniraptora]], a group of theropods which includes [[Dromaeosauridae|dromaeosaurids]] and [[Oviraptorosauria|oviraptorosaurs]], among others.<ref>{{Cite book|last=Paul |first=Gregory S. |author-link=Gregory S. Paul |chapter=Looking for the True Bird Ancestor |year=2002 |title=Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds |url=https://archive.org/details/dinosaursofairev0000paul |location=Baltimore|publisher=Johns Hopkins University Press |isbn=0-8018-6763-0|pages=[https://archive.org/details/dinosaursofairev0000paul/page/171 171]–224}}</ref> As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the [[Liaoning]] Province of northeast China, which demonstrated many small theropod [[feathered dinosaur]]s, contributed to this ambiguity.<ref>{{Cite book |last=Norell |first=Mark |author2=Mick Ellison |year=2005 |title=Unearthing the Dragon: The Great Feathered Dinosaur Discovery |location=New York |publisher=Pi Press |isbn=0-13-186266-9 |url=https://archive.org/details/unearthingdragon00mark }}</ref><ref name="AP-20140731">{{cite news |last=Borenstein |first=Seth |title=Study traces dinosaur evolution into early birds |url=http://apnews.excite.com/article/20140731/us-sci-shrinking-dinosaurs-a5c053f221.html |date=31 July 2014 |agency=Associated Press |access-date=3 August 2014 |archive-url=https://web.archive.org/web/20140808042331/http://apnews.excite.com/article/20140731/us-sci-shrinking-dinosaurs-a5c053f221.html |archive-date=8 August 2014 }}</ref><ref name="SCI-20140731">{{cite journal |title=Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds |date=1 August 2014 |journal=[[Science (journal)|Science]] |volume=345 |issue=6196 |pages=562–566 |doi=10.1126/science.1252243 |last1=Lee |first1=Michael S. Y. |first2=Andrea|last2=Cau |first3=Darren|last3=Naish|first4=Gareth J.|last4=Dyke |bibcode=2014Sci...345..562L |pmid=25082702 }}</ref>

[[File:Anchiornis feathers.jpg|thumb|''[[Anchiornis huxleyi]]'' is an important source of information on the early evolution of birds in the [[Late Jurassic]] period.<ref name="lietal2010">{{cite journal |last1=Li |first1=Q. |last2=Gao |first2=K.-Q. |last3=Vinther |first3=J. |last4=Shawkey |first4=M. D. |last5=Clarke |first5=J. A. |last6=d'Alba |first6=L. |last7=Meng |first7=Q. |last8=Briggs |first8=D. E. G. |last9=Prum |first9=R. O. |year=2010 |name-list-style=amp |title=Plumage color patterns of an extinct dinosaur |journal=[[Science (journal)|Science]] |volume=327 |issue=5971 |pages=1369–1372 |bibcode=2010Sci...327.1369L |doi=10.1126/science.1186290 |pmid=20133521 |url=http://doc.rero.ch/record/210394/files/PAL_E4402.pdf }}</ref>]]
The consensus view in contemporary [[paleontology|palaeontology]] is that the flying theropods, or [[Avialae|avialans]], are the closest relatives of the [[deinonychosaur]]s, which include dromaeosaurids and [[troodontid]]s.<ref name=Xiaotingia>{{cite journal |title=An ''Archaeopteryx''-like theropod from China and the origin of Avialae |date=28 July 2011 |journal=Nature |volume=475 |pages=465–470 |doi=10.1038/nature10288 |issue=7357 |author1=Xing Xu |author2=Hailu You |author3=Kai Du |author4=Fenglu Han |pmid=21796204 }}</ref> Together, these form a group called [[Paraves]]. Some [[basal (phylogenetics)|basal]] members of Deinonychosauria, such as ''[[Microraptor]]'', have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been [[arboreal]], have been able to glide, or both.<ref name="AHTetal07">{{Cite journal|last1=Turner |first1=Alan H. |last2=Pol |first2=D. |last3=Clarke |first3=J. A. |last4=Erickson |first4=G. M. |last5=Norell |first5=M. A. |date=7 September 2007 |title=A basal dromaeosaurid and size evolution preceding avian flight |journal=[[Science (journal)|Science]] |volume=317 |pages=1378–1381 |doi=10.1126/science.1144066 |pmid=17823350 |issue=5843 |bibcode=2007Sci...317.1378T |doi-access=free }}</ref><ref name="xuetal2003">{{Cite journal|date=23 January 2003|title=Four-winged dinosaurs from China|journal=[[Nature (journal)|Nature]]|volume=421|issue=6921|pages=335–340|doi=10.1038/nature01342|pmid=12540892 |last1=Xu |first1=X. |last2=Zhou |first2=Z. |last3=Wang |first3=X. |last4=Kuang |first4=X. |last5=Zhang |first5=F. |last6=Du |first6=X. |bibcode=2003Natur.421..335X |url=http://doc.rero.ch/record/15275/files/PAL_E2574.pdf }}</ref> Unlike ''Archaeopteryx'' and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that the first avialans were [[omnivore]]s.<ref>{{cite web |url=http://the-scientist.com/2011/07/27/on-the-origin-of-birds |title=On the Origin of Birds |access-date=11 June 2012 |author=Luiggi, Christina |date=July 2011 |publisher=The Scientist |archive-url=https://web.archive.org/web/20120616171500/http://the-scientist.com/2011/07/27/on-the-origin-of-birds/ |archive-date=16 June 2012 }}</ref>

The [[Late Jurassic]] ''Archaeopteryx'' is well known as one of the first [[transitional fossil]]s to be found, and it provided support for the [[theory of evolution]] in the late 19th century. ''Archaeopteryx'' was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.<ref name="mayretal2007">{{Cite journal |doi=10.1111/j.1096-3642.2006.00245.x |last1=Mayr |first1=G. |last2=Pohl |first2=B. |last3=Hartman |first3=S. |last4=Peters |first4=D. S. |date=January 2007 |title=The tenth skeletal specimen of ''Archaeopteryx''| journal=Zoological Journal of the Linnean Society |volume=149 |issue=1 |pages=97–116 |doi-access=free}}</ref>

===Early evolution===
{{See also|List of fossil bird genera}}
[[File:Confuciusornis male.jpg|thumb|left|alt= White slab of rock left with cracks and impression of bird feathers and bone, including long paired tail feathers|''[[Confuciusornis sanctus]]'', a Cretaceous bird from China that lived 125&nbsp;million years ago, is the oldest known bird to have a beak.<ref>{{cite book |last1=Ivanov |first1=M. |last2=Hrdlickova |first2=S. |last3=Gregorova |first3=R. |year=2001 |title=The Complete Encyclopedia of Fossils |publisher=Rebo Publishers |location=Netherlands |page=312}}</ref>]]

Over 40% of key traits found in modern birds evolved during the 60&nbsp;million year transition from the earliest [[Avemetatarsalia|bird-line archosaurs]] to the first [[Maniraptoromorpha|maniraptoromorphs]], i.e. the first dinosaurs closer to living birds than to ''[[Tyrannosaurus|Tyrannosaurus rex]]''. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase.<ref name="paleoitalia">{{Cite journal |last=Cau |first=Andrea |date=2018 |title=The assembly of the avian body plan: a 160-million-year long process |url=http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf |url-status=live |journal=Bollettino della Società Paleontologica Italiana |archive-url=https://web.archive.org/web/20180505065903/http://paleoitalia.org/media/u/archives/01_Cau_2018_BSPI_571.pdf |archive-date=5 May 2018}}</ref><ref name="Benton Dhouailly Early Origin">{{cite journal |last1=Benton |first1=Michael J. |last2=Dhouailly |first2=Danielle |last3=Jiang |first3=Baoyu |last4=McNamara |first4=Maria |title=The Early Origin of Feathers |journal=Trends in Ecology & Evolution |date=September 2019 |volume=34 |issue=9 |pages=856–869 |doi=10.1016/j.tree.2019.04.018 |pmid=31164250 |bibcode=2019TEcoE..34..856B |hdl=10468/8068 |hdl-access=free }}</ref> After the appearance of Maniraptoromorpha, the next 40&nbsp;million years marked a continuous reduction of body size and the accumulation of [[Neoteny|neotenic]] (juvenile-like) characteristics. [[Hypercarnivore|Hypercarnivory]] became increasingly less common while braincases enlarged and forelimbs became longer.<ref name="paleoitalia"/> The [[integument]] evolved into complex, [[pennaceous feather]]s.<ref name="Benton Dhouailly Early Origin"/>

The oldest known paravian (and probably the earliest avialan) fossils come from the [[Tiaojishan Formation]] of China, which has been dated to the late [[Jurassic]] period ([[Oxfordian (stage)|Oxfordian]] stage), about 160&nbsp;million years ago. The avialan species from this time period include ''[[Anchiornis huxleyi]]'', ''[[Xiaotingia zhengi]]'', and ''[[Aurornis xui]]''.<ref name="Nature" />

The well-known probable early avialan, ''Archaeopteryx'', dates from slightly later Jurassic rocks (about 155&nbsp;million years old) from [[Germany]]. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial maneuvering.<ref name="zhengetal2013">{{cite journal | last1 = Zheng | first1 = X. | last2 = Zhou | first2 = Z. | last3 = Wang | first3 = X. | last4 = Zhang | first4 = F. | last5 = Zhang | first5 = X. | last6 = Wang | first6 = Y. | last7 = Wei | first7 = G. | last8 = Wang | first8 = S. | last9 = Xu | first9 = X. | date = 15 March 2013 | title = Hind Wings in Basal Birds and the Evolution of Leg Feathers | journal = Science | volume = 339 | issue = 6125| pages = 1309–1312 | doi = 10.1126/science.1228753 | pmid=23493711 | bibcode = 2013Sci...339.1309Z }}</ref>

Avialans diversified into a wide variety of forms during the [[Cretaceous]] period. Many groups retained [[symplesiomorphy|primitive characteristics]], such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves).<ref name="chiappe2007">{{Cite book|last=Chiappe |first=Luis M. |year=2007 |title=Glorified Dinosaurs: The Origin and Early Evolution of Birds |location=Sydney |publisher=University of New South Wales Press |isbn=978-0-86840-413-4}}</ref> Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the [[pygostyle]], an ossification of fused tail vertebrae.<ref name="paleoitalia"/> In the late Cretaceous, about 100&nbsp;million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size.<ref>{{cite journal |last1=Pickrell |first1=John |title=Early birds may have been too hefty to sit on their eggs |journal=Nature |date=22 March 2018 |doi=10.1038/d41586-018-03447-3 }}</ref> Around 95&nbsp;million years ago, they evolved a better sense of smell.<ref>{{cite web | url=http://archive.cosmosmagazine.com/news/birds-survived-dino-extinction-with-keen-senses/ | title=Birds survived dino extinction with keen senses | access-date=11 June 2012 | author=Agency France-Presse | date=April 2011 | publisher=Cosmos Magazine | archive-url=https://web.archive.org/web/20150402124421/http://archive.cosmosmagazine.com/news/birds-survived-dino-extinction-with-keen-senses/ | archive-date=2 April 2015}}</ref>

A third stage of bird evolution starting with [[Ornithothoraces]] (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, [[Keel (bird anatomy)|keeled]] sternum and the [[alula]], and the loss of grasping hands.
<ref name="paleoitalia"/> {{Cladogram|caption=Cladogram following the results of a phylogenetic study by Cau ''et al.'', 2015<ref name=cauetal2015/>|clades=
{{clade| style=font-size:75%;line-height:80%
     |label1=[[Avialae]]
     |1={{clade
        |1=†''[[Anchiornis]]''
        |2={{clade
        |1=†''[[Archaeopteryx]]''
        |2={{clade
        |1=†''[[Xiaotingia]]''
        |2={{clade
           |1=†''[[Rahonavis]]''
            |2={{clade
              |1={{clade
              |1=†''[[Jeholornis]]''
              |2=†''[[Jixiangornis]]'' }}
           |label2=[[Euavialae]]
           |2={{clade
              |1=†''[[Balaur bondoc|Balaur]]''
              |label2=[[Avebrevicauda]]
              |2={{clade
              |1=†''[[Zhongjianornis]]''
              |2={{clade
                 |1=†''[[Sapeornis]]''
                 |label2=[[Pygostylia]]
                 |2={{clade
                     |1={{extinct}}[[Confuciusornithiformes]]
                     |2={{clade
                        |1={{clade
                           |1=†''[[Protopteryx]]''
                           |2=†''[[Pengornis]]'' }}
                     |2=[[Ornithothoraces]] }} }} }} }} }} }} }} }} }} }} }} }}

===Early diversity of bird ancestors===
{{See also|Protobirds|Avialae}}
{{Cladogram|caption=Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis<ref name=Wang2015>{{Cite journal
| doi = 10.1038/ncomms7987
| pmid = 25942493
| title = The oldest record of ornithuromorpha from the early cretaceous of China
| journal = Nature Communications
| volume = 6
| number = 6987
| year = 2015
| last1 = Wang | first1 = M.
| last2 = Zheng | first2 = X.
| last3 = O'Connor | first3 = J. K.
| last4 = Lloyd | first4 = G. T.
| last5 = Wang | first5 = X.
| last6 = Wang | first6 = Y.
| last7 = Zhang | first7 = X.
| last8 = Zhou | first8 = Z. | page=6987
| bibcode = 2015NatCo...6.6987W| pmc = 5426517
}}</ref>

|clades= {{clade| style=font-size:75%;line-height:80%
  |label1=[[Ornithothoraces]]
  |1={{clade
    |1=†[[Enantiornithes]]
    |label2=[[Euornithes]]
    |2={{clade
   |1=†''[[Archaeorhynchus]]''
   |label2=[[Ornithuromorpha]]
   |2={{Clade
      |1=''†[[Patagopteryx]]''
      |2=''†[[Vorona]]''
      |label3=
      |3={{Clade
         |1=''†[[Schizooura]]''
         |label2=
         |2={{Clade
            |1=†[[Hongshanornithidae]]
            |label2=
            |2={{Clade
               |1=''†[[Jianchangornis]]''
               |label2=
               |2={{Clade
                  |1=†[[Songlingornithidae]]
                  |label2=
                  |2={{Clade
                     |1=''†[[Gansus]]''
                     |label2=
                     |2={{Clade
                        |1=''†[[Apsaravis]]''
                        |label2=[[Ornithurae]]
                        |2={{clade
                          |1={{extinct}}[[Hesperornithes]]
                          |2={{clade
                             |1=†''[[Ichthyornis]]''
                             |2={{clade
                                |1=†''[[Vegavis]]''
                                |2='''Aves'''
                     }}
                  }}
               }}
            }}
         }}
      }}
   }}
}} }} }} }} }} }} }}
[[File:Ichthyornis Clean.png|thumb|upright|''[[Ichthyornis]]'', which lived 93&nbsp;million years ago, was the first known prehistoric bird relative preserved with teeth.]]
The first large, diverse lineage of short-tailed avialans to evolve were the [[Enantiornithes]], or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of [[ecological niche]]s, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, Enantiornithes became extinct along with many other dinosaur groups at the end of the [[Mesozoic]] era.<ref name="chiappe2007"/><ref>{{Cite web |last=Elbein |first=Asher |title=Why Do Birds Have Such Skinny Legs? |url=https://www.scientificamerican.com/article/why-do-birds-have-such-skinny-legs/ |access-date=15 February 2024 |website=Scientific American |language=en}}</ref>

Many species of the second major avialan lineage to diversify, the [[Euornithes]] (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithians lacked [[Perching bird|perching]] adaptations and likely included shorebird-like species, waders, and swimming and diving species.<ref name="brusatte2015">{{cite journal |last1=Brusatte |first1=S.L. |last2=O'Connor |first2=J.K. |last3=Jarvis |first3=J.D. |year=2015 |title=The Origin and Diversification of Birds |journal=Current Biology |volume=25 |issue=19 |pages=R888–R898 |doi=10.1016/j.cub.2015.08.003|pmid=26439352 |doi-access=free |bibcode=2015CBio...25.R888B |hdl=10161/11144 |hdl-access=free }}</ref>

The latter included the superficially [[gull]]-like ''[[Ichthyornis]]''<ref>{{Cite journal |last=Clarke |first=Julia A. |year=2004 |title=Morphology, Phylogenetic Taxonomy, and Systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) |journal=Bulletin of the American Museum of Natural History |volume=286 |pages=1–179 |doi=10.1206/0003-0090(2004)286<0001:MPTASO>2.0.CO;2 |hdl=2246/454 |url=http://publication.plazi.org/id/BF3BCD16FFE2FF9AFFB377697C7DB221 }}</ref> and the [[Hesperornithiformes]], which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.<ref name="chiappe2007"/> The early euornithians also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithians retained teeth in other parts of the jaws).<ref name=louchart2011>{{cite journal |last1=Louchart |first1=Antoine |last2=Viriot |first2=Laurent |title=From snout to beak: the loss of teeth in birds |journal=Trends in Ecology & Evolution |date=December 2011 |volume=26 |issue=12 |pages=663–673 |doi=10.1016/j.tree.2011.09.004 |pmid=21978465 |bibcode=2011TEcoE..26..663L }}</ref> Euornithes also included the first avialans to develop true [[pygostyle]] and a fully mobile fan of tail feathers,<ref name=yixianornis>{{cite journal | last1 = Clarke | first1 = J. A. | last2 = Zhou | first2 = Z. | last3 = Zhang | first3 = F. | date = March 2006 | title = Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of ''Yixianornis grabaui'' | journal = Journal of Anatomy | volume = 208 | issue = 3| pages = 287–308 | doi=10.1111/j.1469-7580.2006.00534.x | pmid=16533313 | pmc=2100246}}</ref> which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.<ref name=zhengetal2013/>

A study on [[mosaic evolution]] in the avian skull found that the [[Most recent common ancestor|last common ancestor]] of all Neornithes might have had a beak similar to that of the modern [[hook-billed vanga]] and a skull similar to that of the [[Eurasian golden oriole]]. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor.<ref>{{cite journal | last1 = Felice | first1 = Ryan N. | last2 = Goswami | first2 = Anjali | year = 2018 | title = Developmental origins of mosaic evolution in the avian cranium | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 115 | issue = 3| pages = 555–60 | doi = 10.1073/pnas.1716437115 | pmid = 29279399 | pmc = 5776993 | bibcode = 2018PNAS..115..555F | doi-access = free }}</ref>

===Diversification of modern birds===
{{See also|Sibley–Ahlquist taxonomy of birds|dinosaur classification}}
{{Cladogram|caption=Major groups of modern birds based on [[Sibley-Ahlquist taxonomy]]
|clades={{clade | style=font-size:85%;line-height:100%;width:325px;
|label1='''Aves'''
|1={{clade
 |label1=[[Palaeognathae]]
 |1=([[ratites]] and [[tinamous]])
 |label2= [[Neognathae]]
 |2={{clade
    |2=(all other birds including [[passerine|perching]] birds)
    |label2=[[Neoaves]]
    |label1=[[Galloanserae]]
    |1=([[landfowl]] and [[waterfowl]])
    }}
   }}
}}
}}

Most studies agree on a [[Cretaceous]] age for the most recent common ancestor of modern birds but estimates range from the Early Cretaceous<ref name="Yonezawa2017">{{cite journal |author=Yonezawa, T. |display-authors=et al |date=2017 |title=Phylogenomics and Morphology of Extinct Paleognaths Reveal the Origin and Evolution of the Ratites |journal=Current Biology |volume=27 |number=1 |pages=68–77 |doi=10.1016/j.cub.2016.10.029|pmid=27989673 |doi-access=free |bibcode=2017CBio...27...68Y }}</ref><ref name=divergence>{{cite journal |last1=Lee |first1=M. S. Y. |last2=Cau |first2=A. |last3=Naish |first3=D. |last4=Dyke |first4=G. J. |title=Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves |journal=Systematic Biology |date=May 2014 |volume=63 |issue=3 |pages=442–449 |doi=10.1093/sysbio/syt110 |pmid=24449041 |doi-access=free }}</ref> to the latest Cretaceous.<ref name=Prum2015>{{cite journal | last1 = Prum | first1 = R. O. | display-authors = et al | year = 2015 | title = A comprehensive phylogeny of birds (Aves) using targeted next-generation DNA sequencing | journal = Nature | volume = 526 | issue = 7574 | pages = 569–573 | bibcode = 2015Natur.526..569P | doi = 10.1038/nature15697 | pmid = 26444237 }}</ref><ref name="kuhl2020">{{cite journal |first1=H. |last1=Kuhl |first2=C. |last2=Frankl-Vilches |first3=A. |last3=Bakker |first4=G. |last4=Mayr |first5=G. |last5=Nikolaus |first6=S. T. |last6=Boerno |first7=S. |last7=Klages |first8=B. |last8=Timmermann |first9=M. |last9=Gahr |date=2020 |volume=38 |number=1 |pages=108–127 |title=An unbiased molecular approach using 3'UTRs resolves the avian family-level tree of life |journal=Molecular Biology and Evolution |doi=10.1093/molbev/msaa191 |pmid=32781465 |pmc=7783168 |hdl=21.11116/0000-0007-B72A-C |hdl-access=free }}</ref> Similarly, there is no agreement on whether most of the early diversification of modern birds occurred in the Cretaceous and associated with breakup of the supercontinent [[Gondwana]] or occurred later and potentially as a consequence of the [[Cretaceous–Paleogene extinction event|Cretaceous–Palaeogene extinction event]].<ref name="Ericson">{{Cite journal |last1=Ericson |first1=Per G.P. |year=2006 |title=Diversification of Neoaves: integration of molecular sequence data and fossils |journal=[[Biology Letters]] |volume=2 |issue=4 |pages=543–547 |doi=10.1098/rsbl.2006.0523 |pmid=17148284 |url=http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/neoaves.pdf |last2=Anderson |first2=C. L. |last3=Britton |first3=T. |last4=Elzanowski |first4=A. |last5=Johansson |first5=U. S. |last6=Källersjö |first6=M. |last7=Ohlson |first7=J. I. |last8=Parsons |first8=T. J. |last9=Zuccon |first9=D. |pmc=1834003 |first10=G. |last10=Mayr |display-authors=1 |access-date=4 July 2008 |archive-url=https://web.archive.org/web/20090325235703/http://www.senckenberg.de/files/content/forschung/abteilung/terrzool/ornithologie/neoaves.pdf |archive-date=25 March 2009 }}</ref> This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous [[evolutionary radiation]], while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy).

The discovery in 2005 of ''[[Vegavis]]'' from the [[Maastrichtian]], the last stage of the [[Late Cretaceous]], proved that the diversification of modern birds started before the [[Cenozoic]] era.<ref>{{cite journal |last1=Clarke |first1=Julia A. |last2=Tambussi |first2=Claudia P. |last3=Noriega |first3=Jorge I. |last4=Erickson |first4=Gregory M. |last5=Ketcham |first5=Richard A. |title=Definitive fossil evidence for the extant avian radiation in the Cretaceous |journal=Nature |date=January 2005 |volume=433 |issue=7023 |pages=305–308 |doi=10.1038/nature03150 |pmid=15662422 |bibcode=2005Natur.433..305C |hdl=11336/80763 |hdl-access=free }}</ref> The affinities of an earlier fossil, the possible [[Galliformes|galliform]] ''[[Austinornis]] lentus'', dated to about 85&nbsp;million years ago,<ref>{{cite journal | last1 = Clarke | first1 = J. A. | year = 2004 | title = Morphology, phylogenetic taxonomy, and systematics of ''Ichthyornis'' and ''Apatornis'' (Avialae: Ornithurae) | journal = Bulletin of the American Museum of Natural History | volume = 286 | pages = 1–179 | doi = 10.1206/0003-0090(2004)286<0001:mptaso>2.0.co;2 | hdl = 2246/454 | url = http://publication.plazi.org/id/BF3BCD16FFE2FF9AFFB377697C7DB221 }}</ref> are still too controversial to provide a fossil evidence of modern bird diversification. In 2020, ''[[Asteriornis]]'' from the Maastrichtian was described, it appears to be a close relative of [[Galloanserae]], the earliest diverging lineage within Neognathae.<ref name="Field2020"/>

Attempts to reconcile molecular and fossil evidence using genomic-scale DNA data and comprehensive fossil information have not resolved the controversy.<ref name=Prum2015/><ref name=Jarvis2014/> However, a 2015 estimate that used a new method for calibrating [[molecular clocks]] confirmed that while modern birds originated early in the Late Cretaceous, likely in Western [[Gondwana]], a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event.<ref name=Claramunt2015>{{cite journal |last1=Claramunt |first1=S. |last2=Cracraft |first2=J.|author-link2=Joel Cracraft |title=A new time tree reveals Earth history's imprint on the evolution of modern birds |journal=Sci Adv |date=2015 |volume=1 |issue=11 |doi=10.1126/sciadv.1501005 |pmc=4730849 |pmid=26824065 |article-number=e1501005|bibcode=2015SciA....1E1005C }}</ref> Modern birds would have expanded from West Gondwana through two routes. One route was an Antarctic interchange in the Paleogene. The other route was probably via Paleocene land bridges between South America and North America, which allowed for the rapid expansion and diversification of Neornithes into the [[Holarctic]] and [[Paleotropics]].<ref name=Claramunt2015/> On the other hand, the occurrence of ''Asteriornis'' in the Northern Hemisphere suggest that Neornithes dispersed out of East Gondwana before the Paleocene.<ref name="Field2020"/>

===Classification of bird orders===
{{See also|List of birds}}

All modern birds lie within the [[crown group]] Aves (alternately Neornithes), which has two subdivisions: the [[Palaeognathae]], which includes the flightless [[ratite]]s (such as the [[ostrich]]es) and the weak-flying [[tinamou]]s, and the extremely diverse [[Neognathae]], containing all other birds.<ref name="Mitchell2014">{{Cite journal | doi = 10.1126/science.1251981| pmid = 24855267| title = Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution| journal = Science| volume = 344| issue = 6186| pages = 898–900| date = 23 May 2014| last1 = Mitchell | first1 = K. J.| last2 = Llamas | first2 = B.| last3 = Soubrier | first3 = J.| last4 = Rawlence | first4 = N. J.| last5 = Worthy | first5 = T. H.| last6 = Wood | first6 = J.| last7 = Lee | first7 = M. S. Y.| last8 = Cooper | first8 = A.| bibcode = 2014Sci...344..898M| hdl = 2328/35953 | hdl-access = free}}</ref> These two subdivisions have variously been given the [[taxonomic rank|rank]] of [[superorder]],<ref>{{cite web|url=http://people.eku.edu/ritchisong/birdbiogeography1.htm |title=Bird biogeography | last= Ritchison| first=Gary |access-date=10 April 2008 |work=Avian Biology|publisher=Eastern Kentucky University}}</ref> cohort,<ref name="Theropoda"/> or infraclass.<ref>{{cite book |last1=Cracraft |first1=J. |chapter=Avian Higher-level Relationships and Classification: Nonpasseriforms |pages=xxi–xli |editor-last1=Dickinson |editor-first1=E. C.|editor-first2=J. V. |editor-last2=Remsen|year=2013 |title=The Howard and Moore Complete Checklist of the Birds of the World |edition=4th |volume=1 |publisher=Aves Press, Eastbourne, U.K.}}</ref> The number of known living bird species is around 11,000<ref name="IOC14.2">{{cite taxon|IOC|version=14.2|doi=|doi-access=|access-date=21 November 2024}}</ref><ref>{{Cite web |title=October 2022 {{!}} Clements Checklist |url=https://www.birds.cornell.edu/clementschecklist/updateindex/october-2022/ |access-date=6 January 2023 |website=www.birds.cornell.edu}}</ref> although sources may differ in their precise numbers.

[[Cladogram]] of modern bird relationships based on Stiller ''et al'' (2024).,<ref>{{cite journal |last1=Stiller |first1=Josefin |last2=Feng |first2=Shaohong |last3=Chowdhury |first3=Al-Aabid |last4=Rivas-González |first4=Iker |last5=Duchêne |first5=David A. |last6=Fang |first6=Qi |last7=Deng |first7=Yuan |last8=Kozlov |first8=Alexey |last9=Stamatakis |first9=Alexandros |last10=Claramunt |first10=Santiago |last11=Nguyen |first11=Jacqueline M. T. |last12=Ho |first12=Simon Y. W. |last13=Faircloth |first13=Brant C. |last14=Haag |first14=Julia |last15=Houde |first15=Peter |last16=Cracraft |first16=Joel |last17=Balaban |first17=Metin |last18=Mai |first18=Uyen |last19=Chen |first19=Guangji |last20=Gao |first20=Rongsheng |last21=Zhou |first21=Chengran |last22=Xie |first22=Yulong |last23=Huang |first23=Zijian |last24=Cao |first24=Zhen |last25=Yan |first25=Zhi |last26=Ogilvie |first26=Huw A. |last27=Nakhleh |first27=Luay |last28=Lindow |first28=Bent |last29=Morel |first29=Benoit |last30=Fjeldså |first30=Jon |last31=Hosner |first31=Peter A. |last32=da Fonseca |first32=Rute R. |last33=Petersen |first33=Bent |last34=Tobias |first34=Joseph A. |last35=Székely |first35=Tamás |last36=Kennedy |first36=Jonathan David |last37=Reeve |first37=Andrew Hart |last38=Liker |first38=Andras |last39=Stervander |first39=Martin |last40=Antunes |first40=Agostinho |last41=Tietze |first41=Dieter Thomas |last42=Bertelsen |first42=Mads F. |last43=Lei |first43=Fumin |last44=Rahbek |first44=Carsten |last45=Graves |first45=Gary R. |last46=Schierup |first46=Mikkel H. |last47=Warnow |first47=Tandy |last48=Braun |first48=Edward L. |last49=Gilbert |first49=M. Thomas P. |last50=Jarvis |first50=Erich D. |last51=Mirarab |first51=Siavash |last52=Zhang |first52=Guojie |title=Complexity of avian evolution revealed by family-level genomes |journal=Nature |date=23 May 2024 |volume=629 |issue=8013 |pages=851–860 |doi=10.1038/s41586-024-07323-1 |pmid=38560995 |pmc=11111414 |bibcode=2024Natur.629..851S }}</ref> showing the 44 orders recognised by the IOC.<ref name=IOC14.2/>

{{clade transclude
|page=Template:Phylogeny/Birds
|label=Aves
|wrap='''Aves'''
|nohidden=yes}}

The classification of birds is a contentious issue. [[Charles Sibley|Sibley]] and [[Jon Ahlquist|Ahlquist]]'s ''Phylogeny and Classification of Birds'' (1990) is a landmark work on the subject.<ref>{{Cite book|last=Sibley |first=Charles |author-link=Charles Sibley |author2=Jon Edward Ahlquist |year=1990 |title=Phylogeny and classification of birds |location=New Haven |publisher=Yale University Press |isbn=0-300-04085-7|author2-link=Jon Edward Ahlquist }}</ref> Most evidence seems to suggest the assignment of orders is accurate,<ref>{{Cite book|last=Mayr |first=Ernst |author-link=Ernst W. Mayr |author2=Short, Lester L.|title=Species Taxa of North American Birds: A Contribution to Comparative Systematics |series=Publications of the Nuttall Ornithological Club, no. 9 |year=1970 |publisher=Nuttall Ornithological Club|location=Cambridge, MA |oclc=517185}}</ref> but scientists disagree about the relationships among the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. Fossil and molecular evidence from the 2010s is providing an increasingly clear picture of the evolution of modern bird orders.<ref name=Prum2015/><ref name=Jarvis2014>{{cite journal | last1 = Jarvis | first1 = E. D. | display-authors = et al | year = 2014 | title = Whole-genome analyses resolve early branches in the tree of life of modern birds | journal = Science | volume = 346 | issue = 6215| pages = 1320–1331 | doi=10.1126/science.1253451 | pmid=25504713 | pmc=4405904| bibcode = 2014Sci...346.1320J }}</ref>

===Genomics===
{{See also|list of sequenced animal genomes#Birds}}
In 2010, the [[genome]] had been sequenced for only two birds, the [[chicken]] and the [[zebra finch]]. {{as of|2022}}, the genomes of 542 species of birds had been completed. At least one genome has been sequenced from every order.<ref name="Holmes">{{cite journal |last1=Holmes |first1=Bob |title=Learning about birds from their genomes |journal=Knowable Magazine |date=10 February 2022 |doi=10.1146/knowable-021022-1 |doi-access=free |url=https://knowablemagazine.org/article/living-world/2022/learning-about-birds-their-genomes |access-date=11 February 2022|url-access=subscription }}</ref><ref name="Bravo">{{cite journal |last1=Bravo |first1=Gustavo A. |last2=Schmitt |first2=C. Jonathan |last3=Edwards |first3=Scott V. |title=What Have We Learned from the First 500 Avian Genomes? |journal=Annual Review of Ecology, Evolution, and Systematics |date=3 November 2021 |volume=52 |issue=1 |pages=611–639 |doi=10.1146/annurev-ecolsys-012121-085928 |bibcode=2021AREES..52..611B }}</ref> These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the ''Howard and Moore Checklist'').<ref name="FengStiller2020">{{cite journal|last1=Feng|first1=Shaohong|last2=Stiller|first2=Josefin|last3=Deng|first3=Yuan|last4=Armstrong|first4=Joel|last5=Fang|first5=Qi|last6=Reeve|first6=Andrew Hart|last7=Xie|first7=Duo|last8=Chen|first8=Guangji|last9=Guo|first9=Chunxue|last10=Faircloth|first10=Brant C.|last11=Petersen|first11=Bent|last12=Wang|first12=Zongji|last13=Zhou|first13=Qi|last14=Diekhans|first14=Mark|last15=Chen|first15=Wanjun|last16=Andreu-Sánchez|first16=Sergio|last17=Margaryan|first17=Ashot|last18=Howard|first18=Jason Travis|last19=Parent|first19=Carole|last20=Pacheco|first20=George|last21=Sinding|first21=Mikkel-Holger S.|last22=Puetz|first22=Lara|last23=Cavill|first23=Emily|last24=Ribeiro|first24=Ângela M.|last25=Eckhart|first25=Leopold|last26=Fjeldså|first26=Jon|last27=Hosner|first27=Peter A.|last28=Brumfield|first28=Robb T.|last29=Christidis|first29=Les|last30=Bertelsen|first30=Mads F.|last31=Sicheritz-Ponten|first31=Thomas|last32=Tietze|first32=Dieter Thomas|last33=Robertson|first33=Bruce C.|last34=Song|first34=Gang|last35=Borgia|first35=Gerald|last36=Claramunt|first36=Santiago|last37=Lovette|first37=Irby J.|last38=Cowen|first38=Saul J.|last39=Njoroge|first39=Peter|last40=Dumbacher|first40=John Philip|last41=Ryder|first41=Oliver A.|last42=Fuchs|first42=Jérôme|last43=Bunce|first43=Michael|last44=Burt|first44=David W.|last45=Cracraft|first45=Joel|last46=Meng|first46=Guanliang|last47=Hackett|first47=Shannon J.|last48=Ryan|first48=Peter G.|last49=Jønsson|first49=Knud Andreas|last50=Jamieson|first50=Ian G.|last51=da Fonseca|first51=Rute R.|last52=Braun|first52=Edward L.|last53=Houde|first53=Peter|last54=Mirarab|first54=Siavash|last55=Suh|first55=Alexander|last56=Hansson|first56=Bengt|last57=Ponnikas|first57=Suvi|last58=Sigeman|first58=Hanna|last59=Stervander|first59=Martin|last60=Frandsen|first60=Paul B.|last61=van der Zwan|first61=Henriette|last62=van der Sluis|first62=Rencia|last63=Visser|first63=Carina|last64=Balakrishnan|first64=Christopher N.|last65=Clark|first65=Andrew G.|last66=Fitzpatrick|first66=John W.|last67=Bowman|first67=Reed|last68=Chen|first68=Nancy|last69=Cloutier|first69=Alison|last70=Sackton|first70=Timothy B.|last71=Edwards|first71=Scott V.|last72=Foote|first72=Dustin J.|last73=Shakya|first73=Subir B.|last74=Sheldon|first74=Frederick H.|last75=Vignal|first75=Alain|last76=Soares|first76=André E. R.|last77=Shapiro|first77=Beth|last78=González-Solís|first78=Jacob|last79=Ferrer-Obiol|first79=Joan|last80=Rozas|first80=Julio|last81=Riutort|first81=Marta|last82=Tigano|first82=Anna|last83=Friesen|first83=Vicki|last84=Dalén|first84=Love|last85=Urrutia|first85=Araxi O.|last86=Székely|first86=Tamás|last87=Liu|first87=Yang|last88=Campana|first88=Michael G.|last89=Corvelo|first89=André|last90=Fleischer|first90=Robert C.|last91=Rutherford|first91=Kim M.|last92=Gemmell|first92=Neil J.|last93=Dussex|first93=Nicolas|last94=Mouritsen|first94=Henrik|last95=Thiele|first95=Nadine|last96=Delmore|first96=Kira|last97=Liedvogel|first97=Miriam|last98=Franke|first98=Andre|last99=Hoeppner|first99=Marc P.|display-authors=1|last100=Krone|first100=Oliver|last101=Fudickar|first101=Adam M.|last102=Milá|first102=Borja|last103=Ketterson|first103=Ellen D.|last104=Fidler|first104=Andrew Eric|last105=Friis|first105=Guillermo|last106=Parody-Merino|first106=Ángela M.|last107=Battley|first107=Phil F.|last108=Cox|first108=Murray P.|last109=Lima|first109=Nicholas Costa Barroso|last110=Prosdocimi|first110=Francisco|last111=Parchman|first111=Thomas Lee|last112=Schlinger|first112=Barney A.|last113=Loiselle|first113=Bette A.|last114=Blake|first114=John G.|last115=Lim|first115=Haw Chuan|last116=Day|first116=Lainy B.|last117=Fuxjager|first117=Matthew J.|last118=Baldwin|first118=Maude W.|last119=Braun|first119=Michael J.|last120=Wirthlin|first120=Morgan|last121=Dikow|first121=Rebecca B.|last122=Ryder|first122=T. Brandt|last123=Camenisch|first123=Glauco|last124=Keller|first124=Lukas F.|last125=DaCosta|first125=Jeffrey M.|last126=Hauber|first126=Mark E.|last127=Louder|first127=Matthew I. M.|last128=Witt|first128=Christopher C.|last129=McGuire|first129=Jimmy A.|last130=Mudge|first130=Joann|last131=Megna|first131=Libby C.|last132=Carling|first132=Matthew D.|last133=Wang|first133=Biao|last134=Taylor|first134=Scott A.|last135=Del-Rio|first135=Glaucia|last136=Aleixo|first136=Alexandre|last137=Vasconcelos|first137=Ana Tereza Ribeiro|last138=Mello|first138=Claudio V.|last139=Weir|first139=Jason T.|last140=Haussler|first140=David|last141=Li|first141=Qiye|last142=Yang|first142=Huanming|last143=Wang|first143=Jian|last144=Lei|first144=Fumin|last145=Rahbek|first145=Carsten|last146=Gilbert|first146=M. Thomas P.|last147=Graves|first147=Gary R.|last148=Jarvis|first148=Erich D.|last149=Paten|first149=Benedict|last150=Zhang|first150=Guojie|title=Dense sampling of bird diversity increases power of comparative genomics|journal=Nature|volume=587|issue=7833|year=2020|pages=252–257 |doi=10.1038/s41586-020-2873-9|pmid=33177665|pmc=7759463|bibcode=2020Natur.587..252F|doi-access=free}}</ref>

Being able to sequence and compare whole genomes gives researchers many types of information, about genes, the DNA that regulates the genes, and their evolutionary history. This has led to reconsideration of some of the classifications that were based solely on the identification of protein-coding genes. Waterbirds such as [[pelicans]] and [[flamingos]], for example, may have in common specific adaptations suited to their environment that were developed independently.<ref name="Holmes"/><ref name="Bravo"/>

==Distribution==
{{See also|Lists of birds by region|List of birds by population}}
[[File:House sparrow04.jpg|thumb|left|alt= small bird withpale belly and breast and patterned wing and head stands on concrete |The range of the [[house sparrow]] has expanded dramatically due to human activities.<ref>{{Cite book|last=Newton |first=Ian|year=2003 |title=The Speciation and Biogeography of Birds |location=Amsterdam |publisher=Academic Press |isbn=0-12-517375-X|page=463}}</ref>]]

Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the [[snow petrel]]'s breeding colonies up to {{convert|440|km|mi|-1}} inland in [[Antarctica]].<ref>{{Cite book|last=Brooke |first=Michael |year=2004 |title=Albatrosses And Petrels Across The World |location=Oxford |publisher=Oxford University Press|isbn=0-19-850125-0}}</ref> The highest bird [[biodiversity|diversity]] occurs in tropical regions. It was earlier thought that this high diversity was the result of higher [[speciation]] rates in the tropics; however studies from the 2000s found higher speciation rates in the high latitudes that were offset by greater [[extinction]] rates than in the tropics.<ref>{{Cite journal|last1=Weir |first1=Jason T. |year=2007 |title=The Latitudinal Gradient in Recent Speciation and Extinction Rates of Birds and Mammals |journal=[[Science (journal)|Science]] |volume=315 |issue=5818 |pages=1574–1576 |doi=10.1126/science.1135590 |pmid=17363673 |last2=Schluter |first2=D |bibcode=2007Sci...315.1574W }}</ref> Many species migrate annually over great distances and across oceans; several families of birds have adapted to life both on the world's oceans and in them, and some [[seabird]] species come ashore only to breed,<ref name="Burger">{{Cite book|last=Schreiber |first=Elizabeth Anne |author2=Joanna Burger |year=2001 |title=Biology of Marine Birds |location=Boca Raton |publisher=CRC Press |isbn=0-8493-9882-7}}</ref> while some [[penguin]]s have been recorded diving up to {{convert|300|m|ft|-1}} deep.<ref>{{Cite journal|last1=Sato |first1=Katsufumi |date=1 May 2002|title=Buoyancy and maximal diving depth in penguins: do they control inhaling air volume? |journal=Journal of Experimental Biology |volume=205 |issue=9 |pages=1189–1197 |pmid=11948196 |first2=Y.|last2=Naito|first3=A.|last3=Kato|first4=Y.|last4=Niizuma|first5=Y.|last5=Watanuki|first6=J. B.|last6=Charrassin|first7=C. A.|last7=Bost|first8=Y.|last8=Handrich|first9=Y. |last9=Le Maho|doi=10.1242/jeb.205.9.1189 |url=http://jeb.biologists.org/cgi/content/full/205/9/1189|url-access=subscription}}</ref>

Many bird species have established breeding populations in areas to which they have been [[introduced species|introduced]] by humans. Some of these introductions have been deliberate; the [[ring-necked pheasant]], for example, has been introduced around the world as a [[game bird]].<ref>{{Cite book|last=Hill |first=David |author2=Peter Robertson |year=1988 |title=The Pheasant: Ecology, Management, and Conservation |location=Oxford |publisher=BSP Professional |isbn=0-632-02011-3}}</ref> Others have been accidental, such as the establishment of wild [[monk parakeet]]s in several North American cities after their escape from captivity.<ref>{{cite journal|last=Spreyer |first=Mark F.|author2=Enrique H. Bucher|year=1998|title=Monk Parakeet (Myiopsitta monachus)|journal=The Birds of North America|publisher=Cornell Lab of Ornithology|url=https://bna.birds.cornell.edu/bna/species/322 |doi=10.2173/bna.322 |access-date=13 December 2015|url-access=subscription}}</ref> Some species, including [[cattle egret]],<ref>{{Cite journal|last=Arendt |first=Wayne J. |date=1 January 1988|title=Range Expansion of the Cattle Egret, (''Bubulcus ibis'') in the Greater Caribbean Basin |journal=Colonial Waterbirds |volume=11 |issue=2 |pages=252–262 |doi=10.2307/1521007 |jstor=1521007}}</ref> [[yellow-headed caracara]]<ref>{{Cite book|last=Bierregaard |first=R. O. |year=1994 |chapter=Yellow-headed Caracara |editor=Josep del Hoyo |editor2=Andrew Elliott |editor3=Jordi Sargatal |title=Handbook of the Birds of the World. Volume 2; New World Vultures to Guineafowl |location=Barcelona |publisher=Lynx Edicions |isbn=84-87334-15-6}}</ref> and [[galah]],<ref>{{Cite book|last=Juniper |first=Tony |author2=Mike Parr |year=1998 |title=Parrots: A Guide to the Parrots of the World |location=London |publisher=[[Helm Identification Guides|Christopher Helm]] |isbn=0-7136-6933-0}}</ref> have [[Avian range expansion|spread naturally]] far beyond their original ranges as [[Agriculture|agricultural expansion]] created alternative habitats although modern practices of intensive agriculture have negatively impacted farmland bird populations.<ref>{{cite book |title=Farmland Birds across the World |publisher=Lynx Edicions |year=2010 |isbn=978-84-96553-63-7 |editor1=Weijden, Wouter van der |publication-place=Barcelona |page=4 |editor2=Terwan, Paul |editor3=Guldemond, Adriaan}}</ref>

==Anatomy and physiology==
{{Main|Bird anatomy|}}

{{See also|Egg tooth}}
[[File:Birdmorphology.svg|thumb|upright=1.35|right|External anatomy of a bird (example: [[yellow-wattled lapwing]]):
{{Horizontal ordered list|class=inline
| Beak
| Head
| Iris
| Pupil
| Mantle
| Lesser [[Covert (feather)|coverts]]
| Scapulars
| Median coverts
| Tertials
| Rump
| Primaries
| [[Cloaca#Birds|Vent]]
| Thigh
| Tibio-tarsal articulation
| Tarsus
| Foot
| Tibia
| Belly
| Flanks
| Breast
| Throat
| Wattle
| Eyestripe
}}
]]

Compared with other vertebrates, birds have a [[body plan]] that shows many unusual adaptations, mostly to facilitate [[bird flight|flight]].

===Skeletal system===
{{Main|Bird anatomy#Skeletal system}}

The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the [[respiratory system]].<ref>{{cite web|last=Ehrlich |first=Paul R. |author2=David S. Dobkin |author3=Darryl Wheye |title=Adaptations for Flight |url=http://www.stanford.edu/group/stanfordbirds/text/essays/Adaptations.html |year=1988 |work=Birds of Stanford |publisher=[[Stanford University]] |access-date=13 December 2007}} Based on The Birder's Handbook ([[Paul Ehrlich]], David Dobkin, and Darryl Wheye. 1988. Simon and Schuster, New York.)</ref> The skull bones in adults are fused and do not show [[cranial sutures]].<ref name="Gill">{{Cite book |last=Gill |first=Frank |year=1995 |title=Ornithology |publisher=WH Freeman and Co |location=New York |isbn=0-7167-2415-4 }}</ref> The [[orbit (anatomy)|orbital cavities]] that house the eyeballs are large and separated from each other by a bony [[septum]] (partition). The [[vertebral column|spine]] has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior [[thoracic vertebrae]] and absent in the later vertebrae.<ref>{{Cite news|title=The Avian Skeleton |url=http://www.paulnoll.com/Oregon/Birds/Avian-Skeleton.html |work=paulnoll.com | last=Noll | first=Paul |access-date=13 December 2007}}</ref> The last few are fused with the [[pelvis]] to form the [[synsacrum]].<ref name="Gill"/> The ribs are flattened and the [[sternum]] is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings.<ref>{{Cite news|title=Skeleton of a typical bird |url=http://fsc.fernbank.edu/Birding/skeleton.htm |work=Fernbank Science Center's Ornithology Web |access-date=13 December 2007}}</ref> The wings are more or less developed depending on the species; the only known groups that lost their wings are the [[extinct]] [[moa]] and [[elephant bird]]s.<ref>{{Cite web |url=https://www.nationalgeographic.com/science/phenomena/2014/05/22/the-surprising-closest-relative-of-the-huge-elephant-birds/|archive-url=https://web.archive.org/web/20181214065448/https://www.nationalgeographic.com/science/phenomena/2014/05/22/the-surprising-closest-relative-of-the-huge-elephant-birds/|archive-date=14 December 2018|title=The Surprising Closest Relative of the Huge Elephant Birds|date=22 May 2014|website=Science & Innovation|access-date=6 March 2019}}</ref>

===Excretory system===
Like [[reptile]]s, birds are primarily [[Metabolic waste#Uricotelism|uricotelic]]; that is, their [[kidney]]s extract [[nitrogenous waste]] from their bloodstream and excrete it as [[uric acid]], instead of [[urea]] or [[ammonia]], through the ureters into the intestine. Birds do not have a [[urinary bladder]] or external urethral opening. With the exception of the [[Ostrich#Description|ostrich]], uric acid is excreted along with faeces as a semisolid waste.<ref>{{cite web|last=Ehrlich |first=Paul R. |author2=David S. Dobkin|author3=Darryl Wheye |title=Drinking |url=http://www.stanford.edu/group/stanfordbirds/text/essays/Drinking.html |year=1988 |work=Birds of Stanford |publisher=Stanford University |access-date=13 December 2007}}</ref><ref>{{Cite journal|last1=Tsahar |first1=Ella |title=Can birds be ammonotelic? Nitrogen balance and excretion in two frugivores |journal=Journal of Experimental Biology |volume=208 |issue=6 |pages=1025–1034 |year=2005 |pmid=15767304 |doi=10.1242/jeb.01495 |last2=Martínez Del Rio |first2=C |last3=Izhaki |first3=I |last4=Arad |first4=Z |doi-access=free |bibcode=2005JExpB.208.1025T }}</ref><ref name=coprodeum>{{cite journal | doi= 10.1016/S1095-6433(03)00006-0 | last1= Skadhauge | first1= E | last2= Erlwanger | first2= KH | last3= Ruziwa | first3= SD | last4= Dantzer | first4= V | last5= Elbrønd | first5= VS | last6= Chamunorwa | first6= JP | title= Does the ostrich (''Struthio camelus'') coprodeum have the electrophysiological properties and microstructure of other birds? | journal= Comparative Biochemistry and Physiology A | volume= 134 | issue= 4 | pages= 749–755 | year= 2003 | pmid = 12814783 }}</ref> However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.<ref>{{Cite journal|last1=Preest |first1=Marion R. |date=April 1997 |title=Ammonia excretion by hummingbirds |journal=Nature |volume=386 |issue= 6625|pages=561–562 |doi=10.1038/386561a0 |last2=Beuchat |first2=Carol A. |bibcode=1997Natur.386..561P }}</ref> They also excrete [[creatine]], rather than [[creatinine]] like mammals.<ref name="Gill"/> This material, as well as the output of the intestines, emerges from the bird's [[cloaca]].<ref>{{Cite journal|last1=Mora |first1=J. |year=1965 |title=The regulation of urea-biosynthesis enzymes in vertebrates |journal=[[Biochemical Journal]] |volume=96 |pages=28–35 |pmid=14343146 |last2=Martuscelli |first2=J |last3=Ortiz Pineda |first3=J |last4=Soberon |first4=G |pmc=1206904|issue=1|doi=10.1042/bj0960028 }}</ref><ref>{{Cite journal|last=Packard |first=Gary C.|year=1966 |title=The Influence of Ambient Temperature and Aridity on Modes of Reproduction and Excretion of Amniote Vertebrates |journal=[[The American Naturalist]] |volume=100 |issue=916 |pages=667–682 |doi=10.1086/282459 |jstor=2459303|bibcode=1966ANat..100..667P }}</ref> The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by [[Bird anatomy#Reproduction|joining cloaca]], and females lay eggs from it. In addition, many species of birds regurgitate [[Pellet (ornithology)|pellets]].<ref>{{Cite journal|last=Balgooyen |first=Thomas G. |date=1 October 1971 |title=Pellet Regurgitation by Captive Sparrow Hawks (''Falco sparverius'') |journal=[[Condor (journal)|Condor]] |volume=73 |issue=3 |pages=382–385 |doi=10.2307/1365774 |url=http://sora.unm.edu/sites/default/files/journals/condor/v073n03/p0382-p0385.pdf |via=Searchable Ornithological Research Archive |archive-url=https://web.archive.org/web/20140224142542/http://sora.unm.edu/sites/default/files/journals/condor/v073n03/p0382-p0385.pdf |archive-date=24 February 2014 |jstor=1365774 }}</ref>

It is a common but not universal feature of [[Altriciality|altricial]] [[passerine]] nestlings (born helpless, under constant parental care) that instead of excreting directly into the nest, they produce a [[fecal sac]]. This is a mucus-covered pouch that allows parents to either dispose of the waste outside the nest or to recycle the waste through their own digestive system.<ref name="audubon">{{cite web|url=https://www.audubon.org/news/what-are-fecal-sacs-bird-diapers-basically |first1=Benji |last1=Jones |title=What Are Fecal Sacs? Bird Diapers, Basically|website=Audubon|date=7 August 2018|access-date=17 January 2021}}</ref>
[[File:20240604 eastern phoebe and fledglings south meadows PD209113.jpg|thumb|Adult [[eastern phoebe]] removing [[fecal sac]] of a fledgling]]

===Reproductive system===
Most male birds do not have [[Intromittent organ|intromittent]] penises.<ref>{{Cite book |url=https://books.google.com/books?id=O5lnDwAAQBAJ&pg=RA3-PA513 |title=Encyclopedia of Animal Behavior |date=21 January 2019 |publisher=Academic Press |isbn=978-0-12-813252-4 |language=en}}</ref> Males within [[Palaeognathae]] (with the exception of the [[Kiwi (bird)|kiwi]]s), the [[Anseriformes]] (with the exception of [[screamer]]s), and in rudimentary forms in [[Galliformes]] (but fully developed in [[Cracidae]]) possess a [[bird penis|penis]], which is never present in [[Neoaves]].<ref>{{cite web|last=Yong |first=Ed |url=http://phenomena.nationalgeographic.com/2013/06/06/how-chickens-lost-their-penises-ducks-kept-theirs/ |archive-url=https://web.archive.org/web/20130609052803/http://phenomena.nationalgeographic.com/2013/06/06/how-chickens-lost-their-penises-ducks-kept-theirs/ |archive-date=9 June 2013 |title= How Chickens Lost Their Penises (And Ducks Kept Theirs) |date=6 June 2013 |work=Phenomena: Not Exactly Rocket Science |publisher=National Geographic |access-date=3 October 2013}}</ref><ref>{{cite web |url=http://bcs.whfreeman.com/gill/bcs-pages/body-right_10.asp?s=10000&n=00010&i=99010.06&v=chapter&o=%7C13000%7C00010%7C&ns=undefined |title=Ornithology, 3rd Edition – Waterfowl: Order Anseriformes |access-date=3 October 2013 |archive-url=https://web.archive.org/web/20150622030534/http://bcs.whfreeman.com/gill/bcs-pages/body-right_10.asp?s=10000&n=00010&i=99010.06&v=chapter&o=%7C13000%7C00010%7C&ns=undefined |archive-date=22 June 2015}}</ref> Its length is thought to be related to [[sperm competition]]<ref>{{cite journal |last1=McCracken |first1=Kevin G. |title=The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata) |journal=The Auk |date=2000 |volume=117 |issue=3 |page=820 |doi=10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2 }}</ref> and it fills with lymphatic fluid instead of blood when erect.<ref>{{cite journal |title=Ostrich penis clears up evolutionary mystery |journal=Nature|year=2011 |doi=10.1038/nature.2011.9600 |last1=Marcus |first1=Adam |doi-access=free }}</ref> When not copulating, it is hidden within the [[proctodeum]] compartment within the cloaca, just inside the vent. Female birds have [[Female sperm storage|sperm storage]] tubules<ref>{{Cite journal|last1=Sasanami|first1=Tomohiro|last2=Matsuzaki|first2=Mei|last3=Mizushima|first3=Shusei|last4=Hiyama|first4=Gen|date=2013|title=Sperm Storage in the Female Reproductive Tract in Birds|journal=Journal of Reproduction and Development |volume=59|issue=4|pages=334–338|doi=10.1262/jrd.2013-038 |pmc=3944358|pmid=23965601}}</ref> that allow sperm to remain viable long after copulation, a hundred days in some species.<ref>{{cite journal|last1=Birkhead|first1=T. R.|last2=Møller|first2=P.|year=1993|title=Sexual selection and the temporal separation of reproductive events: sperm storage data from reptiles, birds and mammals|journal=Biological Journal of the Linnean Society|volume=50|issue=4|pages=295–311|doi=10.1111/j.1095-8312.1993.tb00933.x}}</ref> Sperm from multiple males may [[Sperm competition|compete]] through this mechanism. Most female birds have a single [[ovary]] and a single [[oviduct]], both on the left side,<ref name="karger">{{cite journal |last1=Guioli |first1=Silvana |last2=Nandi |first2=Sunil |last3=Zhao |first3=Debiao |last4=Burgess-Shannon |first4=Jessica |last5=Lovell-Badge |first5=Robin |last6=Clinton |first6=Michael |title=Gonadal Asymmetry and Sex Determination in Birds |journal=Sexual Development |date=2014 |volume=8 |issue=5 |pages=227–242 |doi=10.1159/000358406 |pmid=24577119 |doi-access=free |hdl=20.500.11820/13e43ab5-92e5-46f2-a3a1-5a5e4b9cbe33 |hdl-access=free }}</ref> but there are exceptions: species in at least 16 different orders of birds have two ovaries. Even these species, however, tend to have a single oviduct.<ref name="karger" /> It has been speculated that this might be an adaptation to flight, but males have two testes, and it is also observed that the gonads in both sexes decrease dramatically in size outside the breeding season.<ref>{{cite journal |last1=Dawson |first1=Alistair |title=Annual gonadal cycles in birds: Modeling the effects of photoperiod on seasonal changes in GnRH-1 secretion |journal=Frontiers in Neuroendocrinology |date=April 2015 |volume=37 |pages=52–64 |doi=10.1016/j.yfrne.2014.08.004|pmid=25194876 |doi-access=free }}</ref><ref>{{cite journal |last1=Farner |first1=Donald S. |last2=Follett|first2=Brian K. |last3=King |first3=James R. |last4=Morton|first4=Msrtin L. |title=A Quantitative Examination of Ovarian Growth in the White-Crowned Sparrow |journal=The Biological Bulletin |date=February 1966 |volume=130 |issue=1 |pages=67–75 |doi=10.2307/1539953|jstor=1539953 |pmid=5948479 |bibcode=1966BiolB.130...67F |url=https://www.biodiversitylibrary.org/part/9389 }}</ref> Also, terrestrial birds generally have a single ovary, as does the [[platypus]], an egg-laying mammal. A more likely explanation is that the egg develops a shell while passing through the oviduct over a period of about a day, so that if two eggs were to develop at the same time, there would be a risk to survival.<ref name="karger" /> While rare, mostly abortive, [[parthenogenesis]] is not unknown in birds and eggs can be [[Ploidy#Diploid|diploid]], [[automixis|automictic]] and results in male offspring.<ref>{{cite journal |last1=Ramachandran |first1=R |last2=McDaniel |first2=C D |title=Parthenogenesis in birds: a review |journal=Reproduction |date=June 2018 |volume=155 |issue=6 |pages=R245–R257 |doi=10.1530/REP-17-0728 |pmid=29559496 |doi-access=free }}</ref>

Birds are solely [[Gonochorism|gonochoric]],<ref>{{cite book |doi=10.1007/978-4-431-56609-0_14 |quote-page=290 |quote=Mammals and birds are solely gonochoristic. In mammals, the sex is determined by the genetic male heterogametic system (XX–XY). In birds, the female heterogametic ssytem (ZZ-ZW) is used. |chapter=Genetic Control of Sex Determination and Differentiation in Fish |title=Reproductive and Developmental Strategies |series=Diversity and Commonality in Animals |date=2018 |last1=Matsuda |first1=Masaru |pages=289–306 |isbn=978-4-431-56607-6 }}</ref> meaning they have two sexes: either [[female]] or [[male]]. The sex of birds follows the [[ZW sex-determination system|Z and W sex-determination system]], where the [[Egg cell|ovum]] determines the sex of the offspring. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).<ref name="Gill"/> In [[Mammal|mammals]], by contrast, the [[sperm]] determines the sex of the offspring in the [[XY sex-determination system|X and Y sex-determination]] system. A complex system of [[disassortative mating]] with two morphs is involved in the [[white-throated sparrow]] ''Zonotrichia albicollis'', where white- and tan-browed morphs of opposite sex pair, making it appear as if four sexes were involved since any individual is compatible with only a fourth of the population.<ref>{{Cite journal |last1=Tuttle |first1=Elaina M. |last2=Bergland |first2=Alan O. |last3=Korody |first3=Marisa L. |last4=Brewer |first4=Michael S. |last5=Newhouse |first5=Daniel J. |last6=Minx |first6=Patrick |last7=Stager |first7=Maria |last8=Betuel |first8=Adam |last9=Cheviron |first9=Zachary A. |last10=Warren |first10=Wesley C. |last11=Gonser |first11=Rusty A. |last12=Balakrishnan |first12=Christopher N. |date=2016 |title=Divergence and Functional Degradation of a Sex Chromosome-like Supergene |journal=Current Biology |language=en |volume=26 |issue=3 |pages=344–350 |doi=10.1016/j.cub.2015.11.069 |pmc=4747794 |pmid=26804558|bibcode=2016CBio...26..344T }}</ref>  In all species of birds, an individual's sex is determined at fertilisation. One 2007 study claimed to demonstrate [[temperature-dependent sex determination]] among the [[Australian brushturkey]], for which higher temperatures during incubation resulted in a higher female-to-male [[sex ratio]].<ref>{{Cite journal|last=Göth|first=Anne|title=Incubation temperatures and sex ratios in Australian brush-turkey (''Alectura lathami'') mounds |journal=Austral Ecology|year=2007|volume=32|issue=4|pages=278–285 |doi=10.1111/j.1442-9993.2007.01709.x|bibcode=2007AusEc..32..378G }}</ref> This, however, was later proven to not be the case. These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality.<ref>{{cite journal|title=Temperature-dependent sex ratio in a bird|volume=1|issue=1|date=March 2005|pages=31–33 |last1=Göth |first1=A |last2=Booth |first2=DT |journal=Biology Letters |pmc=1629050 |pmid=17148121 |doi=10.1098/rsbl.2004.0247|bibcode=2005BiLet...1...31G}}</ref>

===Respiratory and circulatory systems===
Birds have one of the most complex [[respiratory system]]s of all animal groups.<ref name="Gill"/> Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior [[Parabronchi|air sac]] which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation.<ref>{{Cite journal |last=Maina |first=John N. |date=2007-01-10 |title=Development, structure, and function of a novel respiratory organ, the lung-air sac system of birds: to go where no other vertebrate has gone |journal=Biological Reviews |language=en |volume=81 |issue=4 |pages=545–579 |doi=10.1111/j.1469-185X.2006.tb00218.x|pmid=17038201 }}</ref> Sound production is achieved using the [[syrinx (biology)|syrinx]], a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea;<ref name="Suthers">{{Cite journal|last=Suthers |first=Roderick A. |author2=Sue Anne Zollinger |pmid=15313772 |doi=10.1196/annals.1298.041 |volume=1016 |issue=1 |title=Producing song: the vocal apparatus |date=June 2004 |journal=Ann. N.Y. Acad. Sci. |pages=109–129|bibcode=2004NYASA1016..109S }}</ref> the trachea being elongated in some species, increasing the volume of vocalisations and the perception of the bird's size.<ref name="Fitch">{{Cite journal|last=Fitch |first=W.T. |year=1999 |title=Acoustic exaggeration of size in birds via tracheal elongation: comparative and theoretical analyses |journal=Journal of Zoology |volume=248 |pages=31–48 |article-number=S095283699900504X |doi=10.1017/S095283699900504X}}</ref>

In birds, the main arteries taking blood away from the heart originate from the right [[aortic arches|aortic arch]] (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the [[aorta]].<ref name="Gill"/> The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating [[red blood cells]] in birds retain their [[cell nucleus|nucleus]].<ref>{{Cite journal|last=Scott |first=Robert B. |date=March 1966 |title=Comparative hematology: The phylogeny of the erythrocyte |journal=Annals of Hematology |volume=12 |issue=6 |pages=340–351 |doi=10.1007/BF01632827 |pmid=5325853 }}</ref>

====Heart type and features====
[[File:Didactic model of an avian heart-FMVZ USP-13 (cropped).jpg|thumb|upright=0.65|[[Educational toy|Didactic model]] of an avian heart]]
The avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac. This pericardial sac is filled with a [[serous fluid]] for lubrication.<ref name=Whittow>{{cite book |last1=Whittow |first1=G. |year=2000 |title=Sturkie's Avian Physiology |editor-first1=G. Causey |editor-last1=Whittow |location=San Diego |publisher=Academic Press}}</ref> The heart itself is divided into a right and left half, each with an [[atrium (heart)|atrium]] and [[ventricle (heart)|ventricle]]. The atrium and ventricles of each side are separated by [[atrioventricular valves]] which prevent back flow from one chamber to the next during contraction. Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium.<ref>{{Cite web |last1=Molnar |first1=Charles |last2=Gair |first2=Jane |date=14 May 2015 |title=21.3. Mammalian Heart and Blood Vessels |url=https://opentextbc.ca/biology/chapter/21-3-mammalian-heart-and-blood-vessels/ |language=en}}</ref>

The [[sinoatrial node]] uses calcium to cause a [[Depolarization|depolarising]] [[signal transduction pathway]] from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. The avian heart also consists of muscular arches that are made up of thick bundles of muscular layers. Much like a mammalian heart, the avian heart is composed of [[endocardial]], [[myocardial]] and [[epicardial]] layers.<ref name=Whittow /> The atrium walls tend to be thinner than the ventricle walls, due to the intense ventricular contraction used to pump oxygenated blood throughout the body. Avian hearts are generally larger than mammalian hearts when compared to body mass. This adaptation allows more blood to be pumped to meet the high metabolic need associated with flight.<ref name="Hoagstrom">{{cite journal |last1=Hoagstrom |first1=C.W. |year=2002 |title=Vertebrate Circulation |journal=Magill's Encyclopedia of Science: Animal Life |volume=1 |pages=217–219 |location=Pasadena, California |publisher=Salem Press}}</ref>

====Organisation====
Birds have a very efficient system for diffusing oxygen into the blood; birds have a ten times greater surface area to [[gas exchange]] volume than mammals. As a result, birds have more blood in their capillaries per unit of volume of lung than a mammal.<ref name="Hoagstrom" /> The arteries are composed of thick elastic muscles to withstand the pressure of the ventricular contractions, and become more rigid as they move away from the heart. Blood moves through the arteries, which undergo [[vasoconstriction]], and into arterioles which act as a transportation system to distribute primarily oxygen as well as nutrients to all tissues of the body. As the arterioles move away from the heart and into individual organs and tissues they are further divided to increase surface area and slow blood flow. Blood travels through the arterioles and moves into the capillaries where gas exchange can occur.<ref name=Hill>{{cite book |last1=Hill |first1=Richard W. |year=2012 |title=Animal Physiology |editor-first1=Richard W. |editor-last1=Hill |editor-first2=Gordon A. |editor-last2=Wyse |editor-first3=Margaret |editor-last3=Anderson |edition=Third |pages=647–678 |publisher=Sinauer Associates |location=Sunderland, MA}}</ref>

Capillaries are organised into capillary beds in tissues; it is here that blood exchanges oxygen for carbon dioxide waste. In the capillary beds, blood flow is slowed to allow maximum [[diffusion]] of oxygen into the tissues. Once the blood has become deoxygenated, it travels through venules then veins and back to the heart. Veins, unlike arteries, are thin and rigid as they do not need to withstand extreme pressure. As blood travels through the venules to the veins a funneling occurs called [[vasodilation]] bringing blood back to the heart.<ref name=Hill /> Once the blood reaches the heart, it moves first into the right atrium, then the right ventricle to be pumped through the lungs for further gas exchange of carbon dioxide waste for oxygen. Oxygenated blood then flows from the lungs through the left atrium to the left ventricle where it is pumped out to the body.<ref name=":1" />

===Nervous system===
{{Main|Bird anatomy#Nervous system}}

The [[nervous system]] is large relative to the bird's size.<ref name="Gill" /> The most developed part of the [[Avian brain|brain of birds]] is the one that controls the flight-related functions, while the [[cerebellum]] coordinates movement and the [[cerebrum]] controls behaviour patterns, navigation, mating and [[Bird nest|nest]] building. Most birds have a poor [[olfaction|sense of smell]]<ref>{{Cite book|title=pockets: birds |last=Barbara|first=Taylor|publisher=Dorling Kindersley|year=2004|isbn=0-7513-5176-8|location=UK|page=16}}</ref> with notable exceptions including [[Kiwi (bird)|kiwi]]s,<ref>{{Cite journal |last=Sales |first=James |year=2005 |title=The endangered kiwi: a review |journal=Folia Zoologica |volume=54 |issue=1–2 |pages=1–20 |id={{ProQuest|206353860}} |url=http://www.ivb.cz/folia/54/1-2/01-20.pdf |access-date=15 September 2007 |archive-url=https://web.archive.org/web/20070926005337/http://www.ivb.cz/folia/54/1-2/01-20.pdf |archive-date=26 September 2007 }}</ref> [[New World vulture]]s<ref name="Avian Sense of Smell">{{cite web|last=Ehrlich |first=Paul R. |author2=David S. Dobkin|author3=Darryl Wheye |title=The Avian Sense of Smell |url=http://www.stanford.edu/group/stanfordbirds/text/essays/Avian_Sense.html |year=1988 |work=Birds of Stanford |publisher=Stanford University |access-date=13 December 2007}}</ref> and [[Procellariiformes|tubenoses]].<ref>{{Cite journal |last1=Lequette |first1=Benoit |last2=Verheyden |first2=Christophe |last3=Jouventin |first3=Pierre |date=August 1989 |title=Olfaction in Subantarctic seabirds: Its phylogenetic and ecological significance |journal=The Condor |volume=91 |issue=3 |pages=732–735 |doi=10.2307/1368131 |jstor=1368131 |url=http://sora.unm.edu/sites/default/files/journals/condor/v091n03/p0732-p0735.pdf |archive-url=https://web.archive.org/web/20131225044650/http://sora.unm.edu/sites/default/files/journals/condor/v091n03/p0732-p0735.pdf |archive-date=25 December 2013 }}</ref> The [[Bird vision|avian visual system]] is usually highly developed. Water birds have special flexible lenses, allowing [[Accommodation (eye)|accommodation]] for vision in air and water.<ref name="Gill" /> Some species also have dual [[Fovea centralis|fovea]]. Birds are [[tetrachromacy|tetrachromatic]], possessing [[ultraviolet]] (UV) sensitive [[cone cell]]s in the eye as well as green, red and blue ones.<ref>{{Cite journal|last1=Wilkie |first1=Susan E. |date=February 1998 |title=The molecular basis for UV vision in birds: spectral characteristics, cDNA sequence and retinal localization of the UV-sensitive visual pigment of the budgerigar (Melopsittacus undulatus) |journal=[[Biochemical Journal]] |volume=330 |pages=541–547 |pmid=9461554 |last2=Vissers |first2=P. M. |last3=Das |first3=D. |last4=Degrip |first4=W. J. |last5=Bowmaker |first5=J. K. |last6=Hunt |first6=D. M. |pmc=1219171|issue=Pt 1 |doi=10.1042/bj3300541}}</ref> They also have [[Double cone (biology)|double cones]], likely to mediate [[Monochromacy|achromatic vision]].<ref>{{cite journal|last1=Olsson|first1=Peter|last2=Lind|first2=Olle|last3=Kelber|first3=Almut|last4=Simmons |first4=Leigh|title=Chromatic and achromatic vision: parameter choice and limitations for reliable model predictions|journal=Behavioral Ecology|volume=29|issue=2|year=2018|pages=273–282 |doi=10.1093/beheco/arx133|doi-access=free}}</ref>

[[File:Bird blink-edit.jpg|thumb|left|upright=1.35|The [[nictitating membrane]] as it covers the eye of a [[masked lapwing]]]]
Many birds show plumage patterns in [[ultraviolet]] that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Male [[blue tit]]s have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.<ref>{{Cite journal|last=Andersson|first=S. |author2=J. Ornborg|author3=M. Andersson |title=Ultraviolet sexual dimorphism and assortative mating in blue tits|journal=[[Proceedings of the Royal Society B]] |year=1998 |volume=265 |issue=1395 |pages=445–450 |doi=10.1098/rspb.1998.0315|pmc=1688915}}</ref> Ultraviolet light is also used in foraging—[[kestrel]]s have been shown to search for prey by detecting the UV fluorescent urine trail marks left on the ground by rodents.<ref>{{Cite journal|last1=Viitala |first1=Jussi |year=1995 |journal=Nature |volume=373 |issue=6513 |pages=425–427 |title=Attraction of kestrels to vole scent marks visible in ultraviolet light |doi=10.1038/373425a0 |last2=Korplmäki |first2=Erkki |last3=Palokangas |first3=Pälvl |last4=Koivula |first4=Minna |bibcode=1995Natur.373..425V }}</ref> With the exception of pigeons and a few other species,<ref>{{cite book| first1=Olin Sewall Jr. |last1=Pettingill|year=1985|title=Ornithology in Laboratory and Field. Fifth Edition|publisher=Academic Press|isbn=0-12-552455-2|location=Orlando, FL|page=11|url=https://books.google.com/books?id=livLBAAAQBAJ&pg=PA11}}</ref> the eyelids of birds are not used in blinking. Instead the eye is lubricated by the [[nictitating membrane]], a third eyelid that moves horizontally.<ref>{{Cite journal|last1=Williams |first1=David L. |date=March 2003 |title=Symblepharon with aberrant protrusion of the nictitating membrane in the snowy owl (''Nyctea scandiaca'') |journal=Veterinary Ophthalmology |volume=6 |issue=1 |pages=11–13 |doi=10.1046/j.1463-5224.2003.00250.x |pmid=12641836 |last2=Flach |first2=E}}</ref> The nictitating membrane also covers the eye and acts as a [[contact lens]] in many aquatic birds.<ref name="Gill"/> The bird [[retina]] has a fan shaped blood supply system called the [[Pecten oculi|pecten]].<ref name="Gill"/>

[[Eye]]s of most birds are large, not very round and capable of only limited movement in the orbits,<ref name="Gill"/> typically 10–20°.<ref name=Land_2014>{{cite journal |doi=10.1007/s00359-014-0964-5 |pmid=25398576 |author=Land, M. F. |date=2014 |title=Eye movements of vertebrates and their relation to eye form and function |journal=Journal of Comparative Physiology A |volume=201 |issue=2 |pages=195–214 }}</ref> Birds with eyes on the sides of their heads have a wide [[visual field]], while birds with eyes on the front of their heads, such as owls, have [[binocular vision]] and can estimate the [[depth of field]].<ref name=Land_2014/><ref>{{Cite journal|last1=Martin |first1=Graham R. |year=1999 |title=Visual fields in Short-toed Eagles, ''Circaetus gallicus'' (Accipitridae), and the function of binocularity in birds |journal=Brain, Behavior and Evolution |volume=53 |issue=2 |pages=55–66 |doi=10.1159/000006582 |pmid=9933782|last2=Katzir |first2=G }}</ref> The avian [[ear]] lacks external [[pinna (anatomy)|pinnae]] but is covered by feathers, although in some birds, such as the ''[[Asio]]'', ''[[Horned owl|Bubo]]'' and ''[[Scops owl|Otus]]'' [[owl]]s, these feathers form tufts which resemble ears. The [[inner ear]] has a [[cochlea]], but it is not a spiral as in mammals.<ref>{{Cite journal |last=Saito |first=Nozomu |year=1978 |title=Physiology and anatomy of avian ear |journal=The Journal of the Acoustical Society of America |volume=64 |issue=S1 |page=S3 |doi=10.1121/1.2004193 |bibcode=1978ASAJ...64....3S|doi-access=free }}</ref> Several species have been demonstrated to hear infrasound (below 20&nbsp;Hz)<ref>{{Cite journal |last1=Zeyl |first1=Jeffrey N. |last2=den Ouden |first2=Olivier |last3=Köppl |first3=Christine |last4=Assink |first4=Jelle |last5=Christensen-Dalsgaard |first5=Jakob |last6=Patrick |first6=Samantha C. |last7=Clusella-Trullas |first7=Susana |date=2020 |title=Infrasonic hearing in birds: a review of audiometry and hypothesized structure–function relationships |journal=Biological Reviews |language=en |volume=95 |issue=4 |pages=1036–1054 |doi=10.1111/brv.12596 |pmid=32237036 |issn=1464-7931|doi-access=free }}</ref> and a few cave-dwelling swifts and oilbirds emit ultrasound (above 20&nbsp;kHz) and [[Animal echolocation|echolocate]] in darkness.<ref>{{Cite journal |last1=Brinkløv |first1=Signe |last2=Fenton |first2=M. Brock |last3=Ratcliffe |first3=John M. |date=2013 |title=Echolocation in Oilbirds and swiftlets |journal=Frontiers in Physiology |volume=4 |page=123 |doi=10.3389/fphys.2013.00123 |issn=1664-042X |pmc=3664765 |pmid=23755019 |doi-access=free}}</ref>

===Defence and intraspecific combat===
A few species are able to use chemical defences against predators; some [[Procellariiformes]] can eject an unpleasant [[stomach oil]] against an aggressor,<ref>{{Cite journal|last=Warham |first=John |date=1 May 1977|title=The incidence, function and ecological significance of petrel stomach oils |journal=Proceedings of the New Zealand Ecological Society |volume=24 |pages=84–93 |url=http://www.newzealandecology.org/nzje/free_issues/ProNZES24_84.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.newzealandecology.org/nzje/free_issues/ProNZES24_84.pdf |archive-date=9 October 2022 |url-status=live |issue=3}}</ref> and some species of [[pitohui]]s from [[New Guinea]] have a powerful [[neurotoxin]] in their skin and feathers.<ref>{{Cite journal|last1=Dumbacher |first1=J.P. |date=October 1992 |title=Homobatrachotoxin in the genus ''Pitohui'': chemical defense in birds? |journal=Science |volume=258 |issue=5083 |pages=799–801 |doi=10.1126/science.1439786 |pmid=1439786 |last2=Beehler |first2=BM |last3=Spande |first3=TF |last4=Garraffo |first4=HM |last5=Daly |first5=JW|bibcode=1992Sci...258..799D }}</ref>

A lack of field observations limit our knowledge, but intraspecific conflicts are known to sometimes result in injury or death.<ref name=long>{{cite journal |last1=Longrich |first1=N.R. |last2=Olson |first2=S.L. |title=The bizarre wing of the Jamaican flightless ibis Xenicibis xympithecus: a unique vertebrate adaptation |journal=Proceedings of the Royal Society B: Biological Sciences |date=5 January 2011 |volume=278 |issue=1716 |pages=2333–2337 |doi=10.1098/rspb.2010.2117 |pmid=21208965 |pmc=3119002}}</ref> The screamers ([[Anhimidae]]), some jacanas (''[[Jacana (genus)|Jacana]]'', ''[[Hydrophasianus]]''), the spur-winged goose (''[[Plectropterus]]''), the torrent duck (''[[Merganetta]]'') and nine species of lapwing (''[[Vanellus]]'') use a sharp spur on the wing as a weapon. The steamer ducks (''[[Tachyeres]]''), geese and swans (''[[Anserinae]]''), the solitaire (''[[Pezophaps]]''), sheathbills (''[[Chionis]]''), some guans (''[[Crax]]'') and stone curlews (''[[Burhinus]]'') use a bony knob on the [[alula]]r metacarpal to punch and hammer opponents.<ref name=long/> The jacanas ''[[Actophilornis]]'' and ''[[Irediparra]]'' have an expanded, blade-like radius. The extinct ''[[Xenicibis]]'' was unique in having an elongate forelimb and massive hand which likely functioned in combat or defence as a jointed club or flail. [[Cygnus olor|Swans]], for instance, may strike with the bony spurs and bite when defending eggs or young.<ref name=long/>

===Feathers, plumage, and scales===
{{Main|Feather|Flight feather|Down feather}}

[[File:African Scops owl.jpg|alt=Owl with eyes closed in front of similarly coloured tree trunk partly obscured by green leaves|thumb|left|The [[disruptively patterned]] plumage of the [[African scops owl]] allows it to blend in with its surroundings.]]

Feathers are a feature characteristic of birds (though also present in [[Feathered dinosaurs|some dinosaurs]] not currently considered to be true birds). They facilitate [[bird flight|flight]], provide insulation that aids in [[thermoregulation]], and are used in display, camouflage, and signalling.<ref name="Gill"/> There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called [[Pterylography|pterylae]]. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called [[plumage]], may vary within species by age, [[social status]],<ref>{{Cite journal |last=Belthoff |first=James R. |date=1 August 1994 |title=Plumage Variation, Plasma Steroids and Social Dominance in Male House Finches |journal=The Condor |volume=96 |issue=3 |pages=614–625 |doi=10.2307/1369464 |author2=Dufty |author3=Gauthreaux |url=http://works.bepress.com/james_belthoff/29 |jstor=1369464 |bibcode=1994Condo..96..614B |url-access=subscription |archive-date=13 March 2017 |access-date=27 February 2017 |archive-url=https://web.archive.org/web/20170313042707/https://works.bepress.com/james_belthoff/29/ |url-status=dead }}</ref> and [[sexual dimorphism|sex]].<ref>{{cite web|last=Guthrie| first=R. Dale|title=How We Use and Show Our Social Organs |work=Body Hot Spots: The Anatomy of Human Social Organs and Behavior |url=http://employees.csbsju.edu/lmealey/hotspots/chapter03.htm |access-date=19 October 2007| archive-url = https://web.archive.org/web/20070621225459/http://employees.csbsju.edu/lmealey/hotspots/chapter03.htm| archive-date = 21 June 2007}}</ref>

Plumage is regularly [[moult]]ed; the standard plumage of a bird that has moulted after breeding is known as the "{{Birdgloss|basic plumage|non-breeding}}" plumage, or—in the [[Humphrey–Parkes terminology]]—"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey–Parkes system as "{{Birdgloss|alternate plumage|alternate}}" plumages.<ref>{{Cite journal|last1=Humphrey |first1=Philip S. |date=1 June 1959|title=An approach to the study of molts and plumages |journal=The Auk |volume=76 |pages=1–31 |url=http://sora.unm.edu/sites/default/files/journals/auk/v076n01/p0001-p0031.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/auk/v076n01/p0001-p0031.pdf |archive-date=9 October 2022 |url-status=live |issue=1 |jstor=4081839|last2=Parkes|first2=K. C.|doi=10.2307/4081839 |bibcode=1959Auk....76....1H }}</ref> Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, [[flight feather]]s are replaced one at a time with the innermost {{Birdgloss|primary}} being the first. When the fifth of sixth primary is replaced, the outermost {{Birdgloss|tertiaries}} begin to drop. After the innermost tertiaries are moulted, the {{Birdgloss|secondaries}} starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary {{Birdgloss|coverts}} are moulted in synchrony with the primary that they overlap.<ref name="pettingill">{{Cite book|author=Pettingill Jr. OS|year=1970|title=Ornithology in Laboratory and Field|url=https://archive.org/details/ornithologyinlab0000pett_i2b3|isbn=0-12-552455-2|publisher=Burgess Publishing Co}}</ref>

[[File:Red Lory (Eos bornea)-6.jpg|alt=Red parrot with yellow bill and wing feathers in bill|upright|right|thumb|[[Red lory]] preening]]

A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.<ref name="debeeretal">{{cite book |last1=de Beer |first1=S. J. |last2=Lockwood |first2=G. M. |last3=Raijmakers |first3=J. H. F. S. |last4=Raijmakers |first4=J. M. H. |last5=Scott |first5=W. A. |last6=Oschadleus |first6=H. D. |last7=Underhill |first7=L. G. |year=2001 |url=http://safring.adu.org.za/downloads/ringers-manual.pdf |title=SAFRING Bird Ringing Manual |archive-url=https://web.archive.org/web/20171019032817/http://safring.adu.org.za/downloads/ringers-manual.pdf |archive-date=19 October 2017}}</ref> As a general rule, the tail feathers are moulted and replaced starting with the innermost pair.<ref name="pettingill"/> Centripetal moults of tail feathers are however seen in the [[Phasianidae]].<ref>{{Cite journal|last=Gargallo|first=Gabriel|date=1 June 1994|title=Flight Feather Moult in the Red-Necked Nightjar ''Caprimulgus ruficollis'' |journal=Journal of Avian Biology |volume=25|issue=2|pages=119–124 |doi=10.2307/3677029 |jstor=3677029}}</ref> The centrifugal moult is modified in the tail feathers of [[woodpecker]]s and [[treecreeper]]s, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail.<ref name="pettingill"/><ref>{{Cite journal|last=Mayr |first=Ernst |year=1954 |title=The tail molt of small owls |journal=The Auk |volume=71 |issue=2 |pages=172–178 |url=http://sora.unm.edu/sites/default/files/journals/auk/v071n02/p0172-p0178.pdf |archive-url=https://web.archive.org/web/20141004053953/http://sora.unm.edu/sites/default/files/journals/auk/v071n02/p0172-p0178.pdf |archive-date=4 October 2014 |doi=10.2307/4081571 |jstor=4081571 }}</ref> The general pattern seen in [[passerine]]s is that the primaries are replaced outward, secondaries inward, and the tail from centre outward.<ref>{{cite web|first=Robert B. |last=Payne |title=Birds of the World, Biology 532 |url=http://www.ummz.umich.edu/birds/resources/families_otw.html |publisher=Bird Division, University of Michigan Museum of Zoology |access-date=20 October 2007 |archive-url=https://web.archive.org/web/20120226062512/http://www.ummz.umich.edu/birds/resources/families_otw.html |archive-date=26 February 2012 }}</ref> Before nesting, the females of most bird species gain a bare [[brood patch]] by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation.<ref>{{Cite journal|last=Turner |first=J. Scott |year=1997 |title=On the thermal capacity of a bird's egg warmed by a brood patch |journal=Physiological Zoology |volume=70 |issue=4 |pages=470–480 |doi=10.1086/515854 |pmid=9237308 }}</ref>

Feathers require maintenance and birds [[preening|preen]] or groom them daily, spending an average of around 9% of their daily time on this.<ref>{{Cite journal|last=Walther |first=Bruno A. |year=2005 |title=Elaborate ornaments are costly to maintain: evidence for high maintenance handicaps |journal=Behavioral Ecology |volume=16 |issue=1 |pages=89–95 |doi=10.1093/beheco/arh135|doi-access=free }}</ref> The bill is used to brush away foreign particles and to apply [[wax]]y secretions from the [[uropygial gland]]; these secretions protect the feathers' flexibility and act as an [[Antimicrobial|antimicrobial agent]], inhibiting the growth of feather-degrading [[bacteria]].<ref>{{Cite journal|last1=Shawkey |first1=Matthew D. |year=2003 |title=Chemical warfare? Effects of uropygial oil on feather-degrading bacteria |journal=[[Journal of Avian Biology]] |volume=34 |issue=4 |pages=345–349 |doi=10.1111/j.0908-8857.2003.03193.x |last2=Pillai |first2=Shreekumar R. |last3=Hill |first3=Geoffrey E.}}</ref> This may be supplemented with the secretions of [[formic acid]] from ants, which birds receive through a behaviour known as [[Anting (bird activity)|anting]], to remove feather parasites.<ref>{{Cite journal|last=Ehrlich |first=Paul R. |year=1986 |title=The Adaptive Significance of Anting |journal=The Auk |volume=103 |issue=4 |page=835 |url=http://sora.unm.edu/sites/default/files/journals/auk/v103n04/p0835-p0835.pdf |archive-url=https://web.archive.org/web/20160305202116/http://sora.unm.edu/sites/default/files/journals/auk/v103n04/p0835-p0835.pdf |archive-date=5 March 2016 }}</ref>

The [[Bird anatomy#Scales|scales]] of birds are composed of the same keratin as beaks, claws, and spurs. They are found mainly on the toes and [[metatarsus]], but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of [[kingfisher]]s and [[woodpecker]]s. The scales of birds are thought to be [[Homology (biology)|homologous]] to those of reptiles and mammals.<ref>{{Cite book|last=Lucas |first=Alfred M. |year=1972 |title=Avian Anatomy – integument |location=East Lansing, Michigan |publisher=USDA Avian Anatomy Project, Michigan State University |pages=67, 344, 394–601}}</ref>

===Flight===
{{Main|Bird flight|Flightless birds}}

[[File:Restless flycatcher04.jpg|alt=Black bird with white chest in flight with wings facing down and tail fanned and down pointing| thumb|[[Restless flycatcher]] in the downstroke of flapping flight]]

Most birds can [[Flying and gliding animals|fly]], which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb ([[Bird wing|wing]]) that serves as an [[airfoil|aerofoil]].<ref name="Gill"/>

Wing shape and size generally determine a bird's flight style and performance; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are [[Flightless bird|flightless]], as were many extinct birds.<ref>{{Cite book|last=Roots |first=Clive |year=2006 |title=Flightless Birds |location=Westport |publisher=Greenwood Press |isbn=978-0-313-33545-7}}</ref> Flightlessness often arises in birds on isolated islands, most likely due to limited resources and the absence of [[mammal]]ian land predators.<ref>{{Cite journal|last=McNab |first=Brian K. |date=October 1994 |title=Energy Conservation and the Evolution of Flightlessness in Birds |journal=The American Naturalist |volume=144 |issue=4 |pages=628–642 |doi=10.1086/285697 |jstor=2462941 |bibcode=1994ANat..144..628M }}</ref> Flightlessness is almost exclusively correlated with [[Island gigantism|gigantism]] due to an island's inherent condition of isolation.<ref>{{cite book |doi=10.1016/B978-0-323-99931-1.00012-X |chapter=Dwarfing and gigantism in Quaternary vertebrates |title=Encyclopedia of Quaternary Science |date=2025 |last1=Palombo |first1=Maria Rita |last2=Moncunill-Solé |first2=Blanca |pages=584–608 |isbn=978-0-443-29997-1 }}</ref><ref>{{cite book |doi=10.1016/B978-012226865-6/00517-1 |chapter=Oceanic Islands: Models of Diversity |title=Encyclopedia of Biodiversity |date=2007 |last1=Gillespie |first1=Rosemary G. |pages=1–13 |isbn=978-0-12-226865-6 }}</ref> Although flightless, penguins use similar musculature and movements to "fly" through the water, as do some flight-capable birds such as [[auk]]s, [[shearwater]]s and [[dipper]]s.<ref>{{cite journal |last1=Kovacs |first1=Christopher E. |last2=Meyers |first2=Ron A. |title=Anatomy and histochemistry of flight muscles in a wing-propelled diving bird, the Atlantic Puffin, ''Fratercula arctica'' |journal=Journal of Morphology |date=May 2000 |volume=244 |issue=2 |pages=109–125 |doi=10.1002/(SICI)1097-4687(200005)244:2<109::AID-JMOR2>3.0.CO;2-0 |pmid=10761049 }}</ref>

==Behaviour==
Most birds are [[diurnal animal|diurnal]], but some birds, such as many species of [[owl]]s and [[nightjar]]s, are [[nocturnal]] or [[crepuscular]] (active during twilight hours), and many coastal [[wader]]s feed when the tides are appropriate, by day or night.<ref>{{Cite journal|last1=Robert |first1=Michel |date=January 1989 |title=Conditions and significance of night feeding in shorebirds and other water birds in a tropical lagoon |journal=The Auk |volume=106 |issue=1 |pages=94–101 |doi=10.2307/4087761 |last2=McNeil |first2=Raymond |last3=Leduc |first3=Alain |jstor=4087761 |bibcode=1989Auk...106...94R |url=http://sora.unm.edu/sites/default/files/journals/auk/v106n01/p0094-p0101.pdf |archive-url=https://web.archive.org/web/20141004070208/http://sora.unm.edu/sites/default/files/journals/auk/v106n01/p0094-p0101.pdf |archive-date=4 October 2014}}</ref>

===Diet and feeding===
{{See also|Bird food}}
[[File:BirdBeaksA.svg|thumb|upright=0.6|alt=Illustration of the heads of 16 types of birds with different shapes and sizes of beak|Feeding adaptations in beaks]]
[[File:Paloma (3771094735).jpg|thumb|[[Feral pigeon]] eating bread.]]
{{Birdgloss|dietary classification terms (-vores)|Birds' diets}} are varied and often include [[nectar (plant)|nectar]], fruit, plants, seeds, [[carrion]], and various small animals, including other birds.<ref name="Gill"/> The [[digestive system of birds]] is unique, with a [[Crop (anatomy)|crop]] for storage and a [[gizzard]] that contains swallowed stones for grinding food to compensate for the lack of teeth.<ref>{{Cite journal|last=Gionfriddo |first=James P.|author2=Best|date=1 February 1995|title=Grit Use by House Sparrows: Effects of Diet and Grit Size|journal=Condor|volume=97|issue=1|pages=57–67|doi=10.2307/1368983 |jstor=1368983 |bibcode=1995Condo..97...57G |url=http://sora.unm.edu/sites/default/files/journals/condor/v097n01/p0057-p0067.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/condor/v097n01/p0057-p0067.pdf |archive-date=9 October 2022 |url-status=live}}</ref> Some species such as pigeons and some psittacine species do not have a [[gallbladder]].<ref>{{Cite journal |date=2010|title=Complex Evolution of Bile Salts in Birds|journal=The Auk|doi=10.1525/auk.2010.09155 |pmc=2990222|last1=Hagey|first1=Lee R.|last2=Vidal|first2=Nicolas|last3=Hofmann|first3=Alan F. |last4=Krasowski|first4=Matthew D.|volume=127|issue=4 |pages=820–831|pmid=21113274}}</ref> Most birds are highly adapted for rapid digestion to aid with flight.<ref>{{Cite book|last=Attenborough|first=David |title=The Life of Birds|title-link=The Life of Birds |publisher=Princeton University Press|year=1998 |isbn=0-691-01633-X|location=Princeton|author-link=David Attenborough}}</ref> Some migratory birds have adapted to use protein stored in many parts of their bodies, including protein from the intestines, as additional energy during migration.<ref name="Battley2">{{Cite journal|last1=Battley|first1=Phil F. |last2=Piersma|first2=T|last3=Dietz|first3=MW|last4=Tang|first4=S|last5=Dekinga|first5=A|last6=Hulsman |first6=K|date=January 2000|title=Empirical evidence for differential organ reductions during trans-oceanic bird flight|journal=[[Proceedings of the Royal Society B]]|volume=267|issue=1439|pages=191–195 |doi=10.1098/rspb.2000.0986|pmc=1690512 |pmid=10687826 |bibcode=2000PBioS.267..191B }} (Erratum in ''Proceedings of the Royal Society B'' '''267'''(1461):2567.)</ref>

Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists.<ref name="Gill"/> [[Avian foraging]] strategies can vary widely by species. Many birds [[Gleaning (birds)|glean]] for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek [[Pest (organism)|pest]] [[insect]]s are considered beneficial 'biological control agents' and their presence encouraged in [[biological pest control]] programmes.<ref name="lwa001">{{cite web |url=http://lwa.gov.au/files/products/land-water-and-wool/pf061365/pf061365.pdf |title=Birds on New England wool properties – A woolgrower guide |access-date=17 July 2010 |publisher=Australian Government – Land and Water Australia |work=Land, Water & Wool Northern Tablelands Property Fact Sheet |author=Reid, N. |year=2006 |archive-date=15 March 2011 |archive-url=https://web.archive.org/web/20110315005353/http://lwa.gov.au/files/products/land-water-and-wool/pf061365/pf061365.pdf}}</ref> Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually.<ref>{{cite journal |last1=Nyffeler |first1=M. |last2=Şekercioğlu |first2=Ç. H. |last3=Whelan |first3=C. J. |date=August 2018 |title=Insectivorous birds consume an estimated 400–500 million tons of prey annually |journal=[[The Science of Nature]] |volume=105 |issue=7–8 |page= 47|doi=10.1007/s00114-018-1571-z |pmid=29987431 |pmc=6061143 |bibcode=2018SciNa.105...47N }}</ref>

Nectar feeders such as [[hummingbird]]s, [[sunbird]]s, [[lories and lorikeets|lories, and lorikeets]] amongst others have specially adapted brushy tongues and in many cases bills designed to fit [[Coevolution|co-adapted]] flowers.<ref>{{Cite journal|last1=Paton |first1=D. C. |date=1 April 1989|title=Bills and tongues of nectar-feeding birds: A review of morphology, function, and performance, with intercontinental comparisons |journal=Australian Journal of Ecology |volume=14 |issue=4 |pages=473–506 |doi=10.1111/j.1442-9993.1989.tb01457.x |first2=B.G. |last2=Collins |bibcode=1989AusEc..14..473P }}</ref> [[Kiwi (bird)|Kiwi]]s and [[shorebird]]s with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of [[ecological niche]]s.<ref name="Gill"/><ref>{{Cite journal|last1=Baker |first1=Myron Charles |date=1 April 1973|title=Niche Relationships Among Six Species of Shorebirds on Their Wintering and Breeding Ranges |journal=Ecological Monographs |volume=43 |issue=2 |pages=193–212 |doi=10.2307/1942194 |first2=Ann Eileen Miller |last2=Baker |jstor=1942194|bibcode=1973EcoM...43..193B }}</ref> [[Loon|Diver]]s, [[diving duck]]s, [[penguin]]s and [[auks]] pursue their prey underwater, using their wings or feet for propulsion,<ref name="Burger"/> while aerial predators such as [[sulidae|sulids]], [[kingfisher]]s and [[tern]]s plunge dive after their prey. [[Flamingo]]s, three species of [[prion (bird)|prion]], and some ducks are [[filter feeder]]s.<ref>{{Cite journal|last1=Cherel |first1=Yves |year=2002 |title=Food and feeding ecology of the sympatric thin-billed ''Pachyptila belcheri'' and Antarctic ''P. desolata'' prions at Iles Kerguelen, Southern Indian Ocean |journal=Marine Ecology Progress Series |volume=228 |pages=263–281 |doi=10.3354/meps228263 |last2=Bocher |first2=P |last3=De Broyer |first3=C |last4=Hobson |first4=KA|bibcode=2002MEPS..228..263C |doi-access=free }}</ref><ref>{{Cite journal|last=Jenkin |first=Penelope M. |year=1957 |title=The Filter-Feeding and Food of Flamingoes (Phoenicopteri) |journal=Philosophical Transactions of the Royal Society B |volume=240 |issue=674 |pages=401–493 |doi=10.1098/rstb.1957.0004 |jstor=92549|bibcode=1957RSPTB.240..401J }}</ref> [[Geese]] and [[dabbling duck]]s are primarily grazers.<ref>{{cite book |last1=Hughes |first1=Baz |last2=Green |first2=Andy J. |chapter=Feeding Ecology |editor-last1=Kear |editor-first1= Janet|title=Ducks, Geese and Swans. |publisher=Oxford University Press |year=2005|pages=42–44 |isbn=978-0-19-861008-3 |chapter-url=https://books.google.com/books?id=MfrdBcKd79wC&pg=PA42}}</ref><ref>{{cite journal |title=The Craniolingual Morphology of Waterfowl (Aves, Anseriformes) and Its Relationship with Feeding Mode Revealed Through Contrast-Enhanced X-Ray Computed Tomography and 2D Morphometrics |year=2016 |last1=Li |first1=Zhiheng |last2=Clarke |first2=Julia A. |journal=Evolutionary Biology |volume=43 |issue=1 |pages=12–25 |doi=10.1007/s11692-015-9345-4 |bibcode=2016EvBio..43...12L }}</ref>

Some species, including [[frigatebird]]s, [[gull]]s,<ref>{{cite journal |last1=Takahashi |first1=Akinori |last2=Kuroki |first2=Maki |last3=Niizuma |first3=Yasuaki |last4=Watanuki |first4=Yutaka |title=Parental Food Provisioning Is Unrelated to Manipulated Offspring Food Demand in a Nocturnal Single-Provisioning Alcid, the Rhinoceros Auklet |journal=Journal of Avian Biology |date=December 1999 |volume=30 |issue=4 |page=486 |doi=10.2307/3677021 |jstor=3677021 |bibcode=1999JAvBi..30..486T }}</ref> and [[skua]]s,<ref>{{Cite journal|last=Bélisle |first=Marc |date=1 August 1995|title=Predation and kleptoparasitism by migrating Parasitic Jaegers |journal=The Condor |volume=97 |issue=3 |pages=771–781 |doi=10.2307/1369185 |url=http://sora.unm.edu/sites/default/files/journals/condor/v097n03/p0771-p0781.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/condor/v097n03/p0771-p0781.pdf |archive-date=9 October 2022 |url-status=live|author2=Giroux|jstor=1369185 }}</ref> engage in [[kleptoparasitism]], stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet; a study of [[great frigatebird]]s stealing from [[masked booby|masked boobies]] estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%.<ref>{{cite journal |last1=Vickery |first1=J. A. |title=The Kleptoparasitic Interactions between Great Frigatebirds and Masked Boobies on Henderson Island, South Pacific |url=https://archive.org/details/sim_condor_1994-05_96_2/page/331 |journal=The Condor |date=May 1994 |volume=96 |issue=2 |pages=331–340 |doi=10.2307/1369318 |jstor=1369318 }}</ref> Other birds are [[scavenger]]s; some of these, like [[vulture]]s, are specialised carrion eaters, while others, like gulls, [[corvid]]s, or other birds of prey, are opportunists.<ref>{{Cite journal|last1=Hiraldo |first1=F. C. |year=1991 |title=Unspecialized exploitation of small carcasses by birds |journal=Bird Studies |volume=38 |issue=3 |pages=200–207 |doi=10.1080/00063659109477089 |last2=Blanco |first2=J. C. |last3=Bustamante |first3=J.|bibcode=1991BirdS..38..200H |hdl=10261/47141 |hdl-access=free }}</ref>

===Water and drinking===
Water is needed by many birds although their mode of excretion and lack of [[sweat gland]]s reduces the physiological demands.<ref>{{Cite book|year=2005|url=http://irs.ub.rug.nl/ppn/287916626|isbn=90-367-2378-7 |last=Engel|first=Sophia Barbara|title=Racing the wind: Water economy and energy expenditure in avian endurance flight|publisher=University of Groningen|access-date=25 November 2008|archive-date=5 April 2020|archive-url=https://web.archive.org/web/20200405201429/http://irs.ub.rug.nl/ppn/287916626}}</ref> Some desert birds can obtain their water needs entirely from moisture in their food. Some have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.<ref>{{cite journal |last1=Tieleman |first1=B. Irene |last2=Williams |first2=Joseph B. |title=The Role of Hyperthermia in the Water Economy of Desert Birds |journal=Physiological and Biochemical Zoology |date=January 1999 |volume=72 |issue=1 |pages=87–100 |doi=10.1086/316640 |pmid=9882607 |bibcode=1999PhyBZ..72...87T |hdl= 11370/6edc6940-c2e8-4c96-832e-0b6982dd59c1 |url=https://pure.rug.nl/ws/files/62402849/The_Role_of_Hyperthermia_in_the_Water_Economy_of_Desert_Birds.pdf |hdl-access=free }}</ref> Seabirds can drink seawater and have [[salt gland]]s inside the head that eliminate excess salt out of the nostrils.<ref>{{Cite journal|title=The Salt-Secreting Gland of Marine Birds|last=Schmidt-Nielsen|first=Knut|journal=Circulation|date=1 May 1960|volume=21|pages=955–967|issue=5|doi=10.1161/01.CIR.21.5.955|pmid=14443123 |bibcode=1960Circu..21..955S |doi-access=free}}</ref>

Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the [[Columbidae|pigeon]], [[Estrildidae|finch]], [[Coliidae|mousebird]], [[Turnicidae|button-quail]] and [[Otididae|bustard]] families are capable of sucking up water without the need to tilt back their heads.<ref>{{Cite journal|first=Sara L.|last=Hallager|title=Drinking methods in two species of bustards|journal=Wilson Bull.|volume=106|issue=4|year=1994|pages=763–764|hdl=10088/4338}}</ref> Some desert birds depend on water sources and [[sandgrouse]] are particularly well known for congregating daily at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers.<ref>{{Cite journal|title=Water Transport by Sandgrouse|url=https://archive.org/details/sim_bioscience_1983-06_33_6/page/365|first=Gordon L.|last= MacLean|journal=BioScience|volume=33|issue= 6|date=1 June 1983|pages=365–369|doi=10.2307/1309104|jstor=1309104}}</ref> Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called [[crop milk]] that they provide to their chicks.<ref>{{cite journal|author=Eraud C|author2=Dorie A|author3=Jacquet A|author4=Faivre B|year=2008|title= The crop milk: a potential new route for carotenoid-mediated parental effects| journal= Journal of Avian Biology| volume=39| pages= 247–251| doi= 10.1111/j.0908-8857.2008.04053.x|issue=2|url=https://hal.archives-ouvertes.fr/hal-00294461/file/Eraud2008.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://hal.archives-ouvertes.fr/hal-00294461/file/Eraud2008.pdf |archive-date=9 October 2022 |url-status=live}}</ref>

===Migration===
{{Main|Bird migration}}

[[File:CanadianGeeseFlyingInVFormation.jpg|thumb|A flock of [[Canada geese]] in [[V formation]]]]
Many bird species migrate to take advantage of global differences of [[season]]al temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, [[shorebird]]s, and [[waterbird]]s undertake annual long-distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the [[temperate]] or [[polar region]]s and a non-breeding season in the [[tropical]] regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.<ref name="Battley">{{Cite journal|last1=Battley |first1=Phil F. |date=January 2000 |title=Empirical evidence for differential organ reductions during trans-oceanic bird flight |journal=[[Proceedings of the Royal Society B]] |volume=267 |issue=1439 |pages=191–195 |doi=10.1098/rspb.2000.0986 |pmid=10687826 |last2=Piersma |first2=T. |last3=Dietz |first3=M. W.|last4=Tang |first4=S |last5=Dekinga |first5=A. |last6=Hulsman |first6=K. |pmc=1690512 |bibcode=2000PBioS.267..191B }} (Erratum in ''Proceedings of the Royal Society B'' '''267'''(1461):2567.)</ref><ref name="Klaassen">{{Cite journal|last=Klaassen |first=Marc |date=1 January 1996|title=Metabolic constraints on long-distance migration in birds |journal=Journal of Experimental Biology |volume=199 |issue=1 |pages=57–64 |doi=10.1242/jeb.199.1.57 |pmid=9317335 |bibcode=1996JExpB.199...57K |url=http://jeb.biologists.org/cgi/reprint/199/1/57 |url-access=subscription }}</ref>

Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around {{convert|2500|km|mi|-2|abbr=on}} and shorebirds can fly up to {{convert|4000|km|mi|-2|abbr=on}},<ref name="Gill"/> although the [[bar-tailed godwit]] is capable of non-stop flights of up to {{convert|10200|km|mi|-2|abbr=on}}.<ref>{{Cite news |title=Long-distance Godwit sets new record |url=http://www.birdlife.org/news/news/2007/04/bar-tailed_godwit_journey.html |date=4 May 2007 |publisher=[[BirdLife International]] |access-date=13 December 2007 |archive-date=2 October 2013 |archive-url=https://web.archive.org/web/20131002131732/http://www.birdlife.org/news/news/2007/04/bar-tailed_godwit_journey.html }}</ref> Some [[seabird]]s undertake long migrations, with the longest annual migrations including those of [[Arctic terns]], which were recorded travelling an average of {{convert|70900|km|abbr=on}} between their [[Arctic]] breeding grounds in [[Greenland]] and [[Iceland]] and their wintering grounds in [[Antarctica]], with one bird covering {{convert|81600|km|abbr=on}},<ref>{{cite journal |last1=Egevang |first1=Carsten |last2=Stenhouse |first2=Iain J. |last3=Phillips |first3=Richard A. |last4=Petersen |first4=Aevar |last5=Fox |first5=James W. |last6=Silk |first6=Janet R. D. |year=2010 |title=Tracking of Arctic terns ''Sterna paradisaea'' reveals longest animal migration |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=5 |pages=2078–2081 |doi=10.1073/pnas.0909493107 |doi-access=free |pmid=20080662 |pmc=2836663 |bibcode=2010PNAS..107.2078E }}</ref> and [[sooty shearwater]]s, which nest in [[New Zealand]] and [[Chile]] and make annual round trips of {{convert|64000|km|mi|-2|abbr=on}} to their summer feeding grounds in the North Pacific off Japan, [[Alaska]] and [[California]].<ref>{{Cite journal|last1=Shaffer |first1=Scott A. |year=2006 |title=Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=103 |issue=34 |pages=12799–12802 |doi=10.1073/pnas.0603715103 |pmid= 16908846 |last2=Tremblay |first2=Y.. |last3=Weimerskirch |first3=H. |last4=Scott |first4=D |last5=Thompson |first5=DR |last6=Sagar |first6=PM |last7=Moller |first7=H. |last8=Taylor |first8=G. A. |last9=Foley |first9=D. G.|pmc=1568927|first10=B. A.|last10=Block|first11=D. P.|last11=Costa|display-authors=1 |bibcode=2006PNAS..10312799S |doi-access=free }}</ref>
Other seabirds disperse after breeding, travelling widely but having no set migration route. [[Albatross]]es nesting in the [[Southern Ocean]] often undertake circumpolar trips between breeding seasons.<ref>{{Cite journal|last1=Croxall |first1=John P. |year=2005 |title=Global Circumnavigations: Tracking year-round ranges of nonbreeding Albatrosses |journal=Science |volume=307 |issue=5707 |pages=249–250 |doi=10.1126/science.1106042 |pmid=15653503 |last2=Silk |first2=J. R. |last3=Phillips |first3=R. A. |last4=Afanasyev |first4=V. |last5=Briggs |first5=D. R.|bibcode=2005Sci...307..249C }}</ref>

[[File:Bar-tailed Godwit migration.jpg|alt= A map of the Pacific Ocean with several coloured lines representing bird routes running from New Zealand to Korea|thumb|left|The routes of satellite-tagged [[bar-tailed godwit]]s migrating north from [[New Zealand]]. This species has the longest known non-stop migration of any species, up to {{convert|10200|km|mi|-2|abbr=on}}.]]
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. [[wikt:irruptive|Irruptive]] species such as the boreal [[finch]]es are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability.<ref>{{Cite journal|last=Wilson | first=W. Herbert Jr. |year=1999 |title=Bird feeding and irruptions of northern finches:are migrations short stopped? |journal=North America Bird Bander |volume=24 |issue=4 |pages=113–121 |url=http://sora.unm.edu/sites/default/files/journals/nabb/v024n04/p0113-p0121.pdf |archive-url=https://web.archive.org/web/20140729162642/https://sora.unm.edu/sites/default/files/journals/nabb/v024n04/p0113-p0121.pdf |archive-date=29 July 2014 }}</ref> Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics; others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.<ref>{{Cite journal|last1=Nilsson |first1=Anna L.K. |year=2006 |title=Do partial and regular migrants differ in their responses to weather? |journal=The Auk |volume=123 |issue=2 |pages=537–547 |doi=10.1642/0004-8038(2006)123[537:DPARMD]2.0.CO;2 |last2=Alerstam |first2=Thomas |last3=Nilsson |first3=Jan-Åke |doi-access=free }}</ref> Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.<ref>{{Cite journal|last=Chan |first=Ken |year=2001 |title=Partial migration in Australian landbirds: a review |journal=[[Emu (journal)|Emu]] |volume=101 |issue=4 |pages=281–292 |doi=10.1071/MU00034|bibcode=2001EmuAO.101..281C }}</ref>

[[Altitudinal migration]] is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when [[territory (animal)|the normal territories]] also become inhospitable due to lack of food.<ref>{{Cite journal|last=Rabenold |first=Kerry N. |year=1985 |title=Variation in Altitudinal Migration, Winter Segregation, and Site Tenacity in two subspecies of Dark-eyed Juncos in the southern Appalachians |journal=The Auk |volume=102 |issue=4 |pages=805–819 |url=http://sora.unm.edu/sites/default/files/journals/auk/v102n04/p0805-p0819.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/auk/v102n04/p0805-p0819.pdf |archive-date=9 October 2022 |url-status=live}}</ref> Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. [[True parrots|Parrots]] as a [[Family (taxonomy)|family]] are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.<ref>{{Cite book|last=Collar |first=Nigel J. |year=1997 |chapter=Family Psittacidae (Parrots) |title=Handbook of the Birds of the World |series=Vol. 4: Sandgrouse to Cuckoos |editor=Josep del Hoyo |editor2=Andrew Elliott |editor3=Jordi Sargatal |location=Barcelona |publisher=Lynx Edicions |isbn=84-87334-22-9|title-link=Handbook of the Birds of the World }}</ref>

The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s, a [[Manx shearwater]] released in [[Boston]] in the United States returned to its colony in [[Skomer]], in Wales within 13 days, a distance of {{convert|5150|km|mi|-2|abbr=on}}.<ref>{{Cite journal|last=Matthews |first=G.V.T. |date=1 September 1953 |title=Navigation in the Manx Shearwater |journal=Journal of Experimental Biology |volume=30 |issue=2 |pages=370–396 |doi=10.1242/jeb.30.3.370 |doi-access=free|bibcode=1953JExpB..30..370M }}</ref> Birds navigate during migration using a variety of methods. For [[diurnal animal|diurnal]] migrants, the [[sun]] is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an [[Chronobiology|internal clock]].<ref name="Gill"/> Orientation with the stellar compass depends on the position of the [[constellation]]s surrounding [[Polaris]].<ref>{{Cite journal|last1=Mouritsen |first1=Henrik |last2=Larsen |first2=Ole Næsbye |date=15 November 2001 |title=Migrating songbirds tested in computer-controlled Emlen funnels use stellar cues for a time-independent compass |journal=Journal of Experimental Biology |volume=204 |issue=8 |pages=3855–3865 |pmid=11807103 |doi=10.1242/jeb.204.22.3855 |doi-access=free|bibcode=2001JExpB.204.3855M }}</ref> These are backed up in some species by their ability to sense the Earth's [[geomagnetism]] through specialised [[Photoreceptor cell|photoreceptors]].<ref>{{Cite journal|last1=Deutschlander |first1=Mark E. |last2=Phillips |first2=J. B. |last3=Borland |first3=S. C. |date=15 April 1999 |title=The case for light-dependent magnetic orientation in animals |journal=Journal of Experimental Biology |volume=202 |issue=8 |pages=891–908 |pmid=10085262 |doi=10.1242/jeb.202.8.891 |doi-access=free|bibcode=1999JExpB.202..891D }}</ref>

===Communication===
{{See also|Bird vocalisation}}
{{Listen|filename=Troglodytes aedon - House Wren - XC79974.ogg|title=Bird song|description=Song of the [[Northern house wren|house wren]], a common North American songbird
|filename2=Tooth-billed_Catbird_audio09.ogg|title2=Mimicry|description2=A [[tooth-billed bowerbird]] mimicking a [[spangled drongo]]
|filename3=Picidae pecking on wood.ogg|title3=Drumming|description3=A [[woodpecker]] drumming on wood}}Birds [[Animal communication|communicate]] primarily using visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).

Birds sometimes use plumage to assess and assert social dominance,<ref>{{Cite journal|last=Möller |first=Anders Pape |year=1988 |title=Badge size in the house sparrow ''Passer domesticus''|journal=[[Behavioral Ecology and Sociobiology]] |volume=22 |issue=5 |pages=373–378 |doi=10.1007/BF00295107 |url=https://www.researchgate.net/publication/226147226 |jstor=4600164}}</ref> to display breeding condition in sexually selected species, or to make threatening displays, as in the [[sunbittern]]'s mimicry of a large predator to ward off [[hawk]]s and protect young chicks.<ref>{{Cite journal|last=Thomas |first=Betsy Trent |date=1 August 1990 |title=Nesting Behavior of Sunbitterns (''Eurypyga helias'') in Venezuela |journal=The Condor |volume=92 |issue=3 |pages=576–581 |doi=10.2307/1368675 |url=http://sora.unm.edu/sites/default/files/journals/condor/v092n03/p0576-p0581.pdf |archive-url=https://web.archive.org/web/20160305194240/http://sora.unm.edu/sites/default/files/journals/condor/v092n03/p0576-p0581.pdf |archive-date=5 March 2016 |author2=Strahl |jstor=1368675 }}</ref>

[[File:Stavenn Eurypiga helias 00.jpg|thumb|left|alt=Large brown patterned ground bird with outstretched wings each with a large spot in the centre|The startling display of the [[sunbittern]] mimics a large predator.]]Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds.<ref name = "Gill"/> The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;<ref>{{Cite journal|last=Pickering |first=S. P. C. |year=2001 |title=Courtship behaviour of the Wandering Albatross ''Diomedea exulans'' at Bird Island, South Georgia |journal=Marine Ornithology |volume=29 |issue=1 |pages=29–37 |doi=10.5038/2074-1235.29.1.488 |url=http://www.marineornithology.org/PDF/29_1/29_1_6.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.marineornithology.org/PDF/29_1/29_1_6.pdf |archive-date=9 October 2022 |url-status=live}}</ref> males' breeding success may depend on the quality of such displays.<ref>{{Cite journal|last=Pruett-Jones |first=S.G. |date=1 May 1990|title=Sexual Selection Through Female Choice in Lawes' Parotia, A Lek-Mating Bird of Paradise |url=https://archive.org/details/sim_evolution_1990-05_44_3/page/486 |journal=[[Evolution (journal)|Evolution]] |volume=44 |issue=3 |pages=486–501 |doi=10.2307/2409431 |author2=Pruett-Jones|jstor=2409431 |pmid=28567971 }}</ref>

[[Bird vocalization|Bird calls and songs]], which are produced in the [[Syrinx (biology)|syrinx]], are the major means by which birds communicate with [[sound]]. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.<ref name = "Suthers"/> Calls are used for a variety of purposes, including mate attraction,<ref name = "Gill"/> evaluation of potential mates,<ref>{{cite journal |last1=Genevois |first1=F. |last2=Bretagnolle |first2=V. |title=Male blue petrels reveal their body mass when calling |journal=Ethology Ecology & Evolution |date=September 1994 |volume=6 |issue=3 |pages=377–383 |doi=10.1080/08927014.1994.9522988 |bibcode=1994EtEcE...6..377G }}</ref> bond formation, the claiming and maintenance of territories,<ref name = "Gill"/><ref name=":0" /> the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),<ref>{{Cite journal|last1=Jouventin |first1=Pierre |date=June 1999 |title=Finding a parent in a king penguin colony: the acoustic system of individual recognition |journal=Animal Behaviour |volume=57 |issue=6 |pages=1175–1183 |doi=10.1006/anbe.1999.1086 |pmid=10373249 |last2=Aubin |first2=T |last3=Lengagne |first3=T |bibcode=1999AnBeh..57.1175J }}</ref> and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.<ref>{{Cite journal|last1=Templeton |first1=Christopher N. |year=2005 |title=Allometry of Alarm Calls: Black-Capped Chickadees Encode Information About Predator Size |journal=Science |volume=308 |issue=5730 |pages=1934–1937 |doi=10.1126/science.1108841 |pmid=15976305 |last2=Greene |first2=E |last3=Davis |first3=K|bibcode=2005Sci...308.1934T }}</ref> Some birds also use mechanical sounds for auditory communication. The ''[[Coenocorypha]]'' [[snipe]]s of [[New Zealand]] drive air through their feathers,<ref name = "Miskelly">{{Cite journal|last=Miskelly |first=C. M. |date=July 1987 |title=The identity of the hakawai |journal=Notornis |volume=34 |issue=2 |pages=95–116}}</ref> [[woodpecker]]s drum for long-distance communication,<ref name="DodenhoffStark2001">{{cite journal|last1=Dodenhoff|first1=Danielle J.|last2=Stark|first2=Robert D.|last3=Johnson|first3=Eric V.|title=Do woodpecker drums encode information for species recognition?|journal=The Condor|volume=103|issue=1|year=2001|page=143 |doi=10.1650/0010-5422(2001)103[0143:DWDEIF]2.0.CO;2 |doi-access=free}}</ref> and [[palm cockatoo]]s use tools to drum.<ref>{{Cite journal|last1=Murphy |first1=Stephen |year=2003 |title=The breeding biology of palm cockatoos (''Probosciger aterrimus''): a case of a slow life history |journal=[[Journal of Zoology]] |volume=261 |issue=4 |pages=327–339 |doi=10.1017/S0952836903004175 |last2=Legge |first2=Sarah |last3=Heinsohn |first3=Robert |bibcode=2003JZoo..261..327M }}</ref>

===Flocking and other associations===
[[File:Red-billed quelea flocking at waterhole.jpg|thumb|right|alt= massive flock of tiny birds seen from distance so that birds appear as specks|[[Red-billed quelea]]s, the most numerous species of wild bird,<ref name = "flycatcher">{{Cite book |last=Sekercioglu |first=Cagan Hakki |year=2006 |chapter=Foreword |title=Handbook of the Birds of the World |series=Vol. 11: Old World Flycatchers to Old World Warblers |editor=Josep del Hoyo |editor2=Andrew Elliott |editor3=David Christie |location=Barcelona |publisher=Lynx Edicions |isbn=84-96553-06-X |page=48|title-link=Handbook of the Birds of the World}}</ref> form enormous flocks{{snd}}sometimes tens of thousands strong.]]
While some birds are essentially territorial or live in small family groups, other birds may form large [[flock (birds)|flocks]]. The principal benefits of flocking are [[safety in numbers]] and increased foraging efficiency.<ref name = "Gill"/> Defence against predators is particularly important in closed habitats like forests, where [[ambush predation]] is common and multiple eyes can provide a valuable [[early warning system]]. This has led to the development of many [[mixed-species feeding flock]]s, which are usually composed of small numbers of many species; these flocks provide safety in numbers but increase potential competition for resources.<ref>{{Cite journal|last=Terborgh |first=John |year=2005 |title=Mixed flocks and polyspecific associations: Costs and benefits of mixed groups to birds and monkeys |journal=American Journal of Primatology |volume=21 |issue=2|pages=87–100 |doi=10.1002/ajp.1350210203|pmid=31963979 }}</ref> Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.<ref>{{Cite journal|last=Hutto |first=Richard L. |date=January 1988|title=Foraging Behavior Patterns Suggest a Possible Cost Associated with Participation in Mixed-Species Bird Flocks |journal=[[Oikos (journal)|Oikos]] |volume=51 |issue=1 |pages=79–83 |doi=10.2307/3565809 |jstor=3565809|bibcode=1988Oikos..51...79H }}</ref> Some species have a mixed system with breeding pairs maintaining territories, while unmated or young birds live in flocks where they secure mates prior to finding territories.<ref>{{Cite journal |last1=Sundar |first1=K. S. Gopi |last2=Grant |first2=John D. A. |last3=Veltheim |first3=Inka |last4=Kittur |first4=Swati |last5=Brandis |first5=Kate |last6=McCarthy |first6=Michael A. |last7=Scambler |first7=Elinor |date=2019 |title=Sympatric cranes in northern Australia: abundance, breeding success, habitat preference and diet |journal=Emu - Austral Ornithology |volume=119 |issue=1 |pages=79–89 |doi=10.1080/01584197.2018.1537673 |bibcode=2019EmuAO.119...79S |hdl=1959.4/unsworks_52806 |hdl-access=free }}</ref>

Birds sometimes also form associations with non-avian species. Plunge-diving [[seabird]]s associate with [[dolphin]]s and [[tuna]], which push shoaling fish towards the surface.<ref name = "AU">{{Cite journal|last=Au |first=David W.K. |date=1 August 1986|title=Seabird interactions with Dolphins and Tuna in the Eastern Tropical Pacific |journal=The Condor |volume=88 |issue=3 |pages=304–317 |url=http://sora.unm.edu/sites/default/files/journals/condor/v088n03/p0304-p0317.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://sora.unm.edu/sites/default/files/journals/condor/v088n03/p0304-p0317.pdf |archive-date=9 October 2022 |url-status=live |doi=10.2307/1368877|author2=Pitman|jstor=1368877 }}</ref> Some species of [[hornbill]]s have a [[Mutualism (biology)|mutualistic relationship]] with [[dwarf mongoose]]s, in which they forage together and warn each other of nearby [[birds of prey]] and other predators.<ref>{{Cite journal|last1=Anne |first1=O. |date=June 1983 |title=Dwarf mongoose and hornbill mutualism in the Taru desert, Kenya |journal=Behavioral Ecology and Sociobiology |volume=12 |issue=3 |pages=181–190 |doi=10.1007/BF00290770 |last2=Rasa |first2=E.|bibcode=1983BEcoS..12..181A }}</ref>

===Resting and roosting===
<!--Roosting redirects here-->
{{Redirect|Roosting||Roost (disambiguation){{!}}Roost}}
[[File:Caribbean Flamingo2 (Phoenicopterus ruber) (0424) - Relic38.jpg|thumb|left|alt=Pink flamingo with grey legs and long neck pressed against body and head tucked under wings|Many birds, like this [[American flamingo]], tuck their head into their back when sleeping.]]
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. [[Avian sleep|Sleeping birds]] often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats.<ref>{{cite journal |last1=Gauthier-Clerc |first1=Michel |last2=Tamisier |first2=Alain |last3=Cézilly |first3=Frank |title=Sleep-Vigilance Trade-Off in Gadwall During the Winter Period |journal=The Condor |date=2000 |volume=102 |issue=2 |page=307 |doi=10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2 |jstor=1369642 }}</ref> [[Swift (bird)|Swift]]s are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight.<ref>{{Cite journal|journal=The Journal of Experimental Biology|volume=205|pages=905–910|date=1 April 2002|title=Harmonic oscillatory orientation relative to the wind in nocturnal roosting flights of the swift ''Apus apus''|first=Johan|last=Bäckman|url=http://jeb.biologists.org/cgi/content/full/205/7/905|pmid=11916987|issue=7|author2=A|doi=10.1242/jeb.205.7.905|bibcode=2002JExpB.205..905B |url-access=subscription}}</ref> It has been suggested that there may be certain kinds of sleep which are possible even when in flight.<ref>{{Cite journal|last=Rattenborg|first=Niels C. |year=2006 |title=Do birds sleep in flight? |journal=Die Naturwissenschaften |volume=93 |issue=9 |pages=413–425 |doi=10.1007/s00114-006-0120-3|pmid=16688436|bibcode=2006NW.....93..413R }}</ref>

Some birds have also demonstrated the capacity to fall into [[slow-wave sleep]] one [[Cerebral hemisphere|hemisphere]] of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for [[predator]]s by viewing the outer margins of the flock. This adaptation is also known from [[marine mammal]]s.<ref>{{Cite journal|last=Milius |first=S. |date=6 February 1999|title=Half-asleep birds choose which half dozes |journal=Science News Online |volume=155 |issue= 6|page=86 |doi=10.2307/4011301 |jstor=4011301 }}</ref> [[Communal roosting]] is common because it lowers the [[thermoregulation|loss of body heat]] and decreases the risks associated with predators.<ref>{{Cite journal|last=Beauchamp |first=Guy |year=1999 |title=The evolution of communal roosting in birds: origin and secondary losses |journal=Behavioral Ecology |volume=10 |issue=6 |pages=675–687 |doi=10.1093/beheco/10.6.675 |doi-access=free }}</ref> Roosting sites are often chosen with regard to thermoregulation and safety.<ref>{{Cite journal|last=Buttemer |first=William A.|year=1985 |title=Energy relations of winter roost-site utilization by American goldfinches (''Carduelis tristis'') |journal=[[Oecologia]] |volume=68 |issue=1 |pages=126–132 |doi=10.1007/BF00379484 |pmid=28310921 |bibcode=1985Oecol..68..126B |hdl=2027.42/47760 |hdl-access=free }}</ref> Unusual mobile roost sites include large herbivores on the African savanna that are used by [[oxpecker]]s.<ref>{{Cite journal|last1=Palmer|first1=Meredith S.|last2=Packer|first2=Craig|date=2018|title=Giraffe bed and breakfast: Camera traps reveal Tanzanian yellow-billed oxpeckers roosting on their large mammalian hosts|journal=African Journal of Ecology |volume=56|issue=4|pages=882–884 |doi=10.1111/aje.12505 |doi-access=free|bibcode=2018AfJEc..56..882P }}</ref>

Many sleeping birds bend their heads over their backs and tuck their [[beak|bills]] in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. [[Passerine|Perching birds]] have a tendon-locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus ''[[Loriculus]]'' roost hanging upside down.<ref>{{Cite journal|last=Buckley |first=F.G. |date=1 January 1968|title=Upside-down Resting by Young Green-Rumped Parrotlets (''Forpus passerinus'') |url=https://archive.org/details/sim_condor_1968-01_70_1/page/89 |journal=The Condor |volume=70 |issue=1 |page=89 |doi=10.2307/1366517 |author2=Buckley|jstor=1366517 }}</ref> Some [[hummingbird]]s go into a nightly state of [[torpor]] accompanied with a reduction of their metabolic rates.<ref>{{Cite journal|last=Carpenter |first=F. Lynn |year=1974 |title=Torpor in an Andean Hummingbird: Its Ecological Significance |journal=Science |volume=183 |issue=4124 |pages=545–547 |doi=10.1126/science.183.4124.545 |pmid=17773043 |bibcode=1974Sci...183..545C }}</ref> This [[Adaptation|physiological adaptation]] shows in nearly a hundred other species, including [[owlet-nightjar]]s, [[nightjar]]s, and [[woodswallow]]s. One species, the [[common poorwill]], even enters a state of [[hibernation]].<ref>{{Cite journal|last1=McKechnie |first1=Andrew E. |year=2007 |title=Torpor in an African caprimulgid, the freckled nightjar ''Caprimulgus tristigma'' |journal=Journal of Avian Biology |volume=38 |issue=3 |pages=261–266 |doi=10.1111/j.2007.0908-8857.04116.x |last2=Ashdown |first2=Robert A.M. |last3=Christian |first3=Murray B. |last4=Brigham |first4=R. Mark |bibcode=2007JAvBi..38..261M }}</ref> Birds do not have sweat glands, but can lose water directly through the skin, and they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or using special behaviours like [[urohidrosis]] to cool themselves.<ref>{{cite book|pages=390–396|title=Ornithology|edition=4| author1=Gill, Frank B.| author2=Prum, Richard O. |publisher=W.H. Freeman|place= New York|year=2019 }}</ref><ref>{{Cite journal |last1=Cabello-Vergel |first1=Julián |last2=Soriano-Redondo |first2=Andrea |last3=Villegas |first3=Auxiliadora |last4=Masero |first4=José A. |last5=Guzmán |first5=Juan M. Sánchez |last6=Gutiérrez |first6=Jorge S. |date=2021 |title=Urohidrosis as an overlooked cooling mechanism in long-legged birds |journal=Scientific Reports |volume=11 |issue=1 |page=20018 |doi=10.1038/s41598-021-99296-8 |pmc=8501033 |pmid=34625581 |bibcode=2021NatSR..1120018C}}</ref>

===Breeding===
{{See also|Category:Avian sexuality|Animal sexual behaviour#Birds|Seabird breeding behaviour|Sexual selection in birds}}

====Social systems====
[[File:Raggiana Bird-of-Paradise wild 5.jpg|thumb|alt= Bird faces up with green face, black breast and pink lower body. Elaborate long feathers on the wings and tail.|right|Like others of its family, the male [[Raggiana bird-of-paradise]] has elaborate breeding plumage used to impress females.<ref>{{Cite journal|doi=10.1071/MU9810193|last=Frith|first=C. B.|title=Displays of Count Raggi's Bird-of-Paradise ''Paradisaea raggiana'' and congeneric species |journal=Emu|volume=81|issue=4|pages=193–201| url=http://www.publish.csiro.au/paper/MU9810193.htm|year=1981|bibcode=1981EmuAO..81..193F |url-access=subscription}}</ref>]]
95 per cent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate.<ref>{{Cite journal |last=Freed|first=Leonard A.|year=1987|title=The Long-Term Pair Bond of Tropical House Wrens: Advantage or Constraint?|journal=[[The American Naturalist]]|volume=130|issue=4|pages=507–525|doi=10.1086/284728|bibcode=1987ANat..130..507F }}</ref> Monogamy allows for both [[paternal care]] and [[Parental investment|biparental care]], which is especially important for species in which care from both the female and the male parent is required in order to successfully rear a brood.<ref>{{Cite journal|last=Gowaty|first=Patricia A.|title=Male Parental Care and Apparent Monogamy among Eastern Bluebirds (''Sialia sialis'')|journal=[[The American Naturalist]]|volume=121|issue=2|pages=149–160 |year=1983|doi=10.1086/284047|bibcode=1983ANat..121..149G }}</ref> Among many socially monogamous species, [[extra-pair copulation]] (infidelity) is common.<ref>{{Cite journal|last1=Westneat|first1=David F.|year=2003|title=Extra-pair paternity in birds: Causes, correlates, and conflict|doi=10.1146/annurev.ecolsys.34.011802.132439|journal=[[Annual Review of Ecology, Evolution, and Systematics]]|volume=34|pages=365–396|last2=Stewart|first2=Ian R.K.}}</ref> Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of [[forced copulation]] in ducks and other [[anatidae|anatids]].<ref>{{Cite journal|last1=Gowaty|first1=Patricia A. |last2=Buschhaus|first2=Nancy|year=1998|title=Ultimate causation of aggressive and forced copulation in birds: Female resistance, the CODE hypothesis, and social monogamy|journal=[[American Zoologist]]|volume=38|issue=1|pages=207–225|doi=10.1093/icb/38.1.207|doi-access=free}}</ref>

For females, possible benefits of extra-pair copulation include getting better genes for her offspring and insuring against the possibility of infertility in her mate.<ref>{{Cite journal|last=Sheldon|first=B|year=1994|title=Male Phenotype, Fertility, and the Pursuit of Extra-Pair Copulations by Female Birds|journal=[[Proceedings of the Royal Society B]]|volume=257|issue=1348|pages=25–30|doi=10.1098/rspb.1994.0089|bibcode=1994RSPSB.257...25S }}</ref> Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.<ref>{{Cite journal|last1=Wei|first1=G|year=2005 |title=Copulations and mate guarding of the Chinese Egret |doi=10.1675/1524-4695(2005)28[527:CAMGOT]2.0.CO;2|journal=Waterbirds|volume=28|issue=4|pages=527–530|last2=Zuo-Hua|first2=Yin|last3=Fu-Min|first3=Lei }}</ref>

Other mating systems, including [[Polygyny in animals|polygyny]], [[Polyandry in animals|polyandry]], [[Animal sexual behaviour#Polygamy|polygamy]], [[polygynandry]], and [[Promiscuity#Other animals|promiscuity]], also occur.<ref name = "Gill"/> Polygamous breeding systems arise when females are able to raise broods without the help of males.<ref name = "Gill"/> Mating systems vary across bird families<ref>{{Cite journal |last1=Owens |first1=Ian P. F. |last2=Bennett |first2=Peter M. |date=1997 |title=Variation in mating system among birds: ecological basis revealed by hierarchical comparative analysis of mate desertion |journal=Proceedings of the Royal Society of London. Series B: Biological Sciences |language=en |volume=264 |issue=1385 |pages=1103–1110 |doi=10.1098/rspb.1997.0152 |pmc=1688567 |bibcode=1997PBioS.264.1103O }}</ref> but variations within species are thought to be driven by environmental conditions.<ref>{{Cite journal |last1=Petrie |first1=Marion |last2=Kempenaers |first2=Bart |date=1998 |title=Extra-pair paternity in birds: explaining variation between species and populations |journal=Trends in Ecology & Evolution |language=en |volume=13 |issue=2 |pages=52–58 |doi=10.1016/S0169-5347(97)01232-9|pmid=21238200 |bibcode=1998TEcoE..13...52P }}</ref> A unique system is the formation of trios where a third individual is allowed by a breeding pair temporarily into the territory to assist with brood raising thereby leading to higher fitness.<ref>{{Cite journal |last1=Barve |first1=Sahas |last2=Riehl |first2=C. |last3=Walters |first3=E. L. |last4=Haydock |first4=J. |last5=Dugdale |first5=H. L. |last6=Koenig |first6=W. D. |date=2021 |title=Lifetime Reproductive Benefits of Cooperative Polygamy Vary for Males and Females in the Acorn Woodpecker (Melanerpes formicivorus). |journal=Proceedings of the Royal Society B: Biological Sciences |volume=208 |issue=1957 |article-number=20210579|doi=10.1098/rspb.2021.0579 |pmid=34403633 |pmc=8370801 }}</ref><ref name=":0">{{Cite journal |last1=Roy |first1=Suhridam |last2=Kittur |first2=Swati |last3=Sundar |first3=K. S. Gopi |date=2022 |title=Sarus crane Antigone antigone trios and their triets: Discovery of a novel social unit in cranes |journal=Ecology |volume=103 |issue=6 |article-number=e3707 |doi=10.1002/ecy.3707 |pmid=35357696 |bibcode=2022Ecol..103E3707R }}</ref>

Breeding usually involves some form of courtship display, typically performed by the male.<ref>{{Cite book|last=Short|first=Lester L.|year=1993|title=Birds of the World and their Behavior|publisher=Henry Holt and Co|location=New York|isbn=0-8050-1952-9|url=https://archive.org/details/livesofbirdsbird00shor}}</ref> Most displays are rather simple and involve some type of [[bird vocalization|song]]. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal [[Lek mating|lekking]]. Females are generally the ones that drive partner selection,<ref>{{Cite book|last=Burton|first=R|year=1985 |title=Bird Behavior|publisher=Alfred A. Knopf, Inc|isbn=0-394-53957-5|url=https://archive.org/details/birdbehavior0000burt}}</ref> although in the polyandrous [[phalaropes]], this is reversed: plainer males choose brightly coloured females.<ref>{{Cite journal|last1=Schamel|first1=D|year=2004|title=Mate guarding, copulation strategies and paternity in the sex-role reversed, socially polyandrous red-necked phalarope ''Phalaropus lobatus'' |journal=Behavioral Ecology and Sociobiology|volume=57|issue=2|pages=110–118|doi=10.1007/s00265-004-0825-2|last2=Tracy |first2=Diane M.|last3=Lank|first3=David B.|last4=Westneat|first4=David F.|bibcode=2004BEcoS..57..110S }}</ref> [[Courtship feeding]], [[Billing (birds)|billing]] and {{Birdgloss|allopreening}} are commonly performed between partners, generally after the birds have paired and mated.<ref>{{Cite book |last=Attenborough |first=David |author-link=David Attenborough |year=1998 |title=The Life of Birds |location=Princeton |publisher=Princeton University Press |isbn=0-691-01633-X |title-link=The Life of Birds }}</ref>

[[Homosexuality in animals#Birds|Homosexual behaviour has been observed]] in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks.<ref>{{cite book |last1=Bagemihl |first1=Bruce |title=Biological exuberance: Animal homosexuality and natural diversity |location=New York |publisher=St. Martin's |year=1999 |pages=479–655}}</ref> Over 130 avian species around the world engage in sexual interactions between the same sex or homosexual behaviours. "Same-sex courtship activities may involve elaborate displays, synchronised dances, gift-giving ceremonies, or behaviours at specific display areas including bowers, arenas, or leks."<ref>{{cite journal |last1=MacFarlane |first1=Geoff R. |last2=Blomberg |first2=Simon P. |last3=Kaplan |first3=Gisela |last4=Rogers |first4=Lesley J. |title=Same-sex sexual behavior in birds: expression is related to social mating system and state of development at hatching |journal=Behavioral Ecology |date=January 2007 |volume=18 |issue=1 |pages=21–33 |doi=10.1093/beheco/arl065 |hdl=10.1093/beheco/arl065 |hdl-access=free }}</ref>

====Territories, nesting and incubation====
{{See also|Bird nest}}
[[File:Bird-nest (2).jpg|alt=two unused bird nest|thumb|left|A bird nest which fell from a tree.]]
Many birds actively defend a territory from others of the same species during the breeding season; maintenance of territories protects the food source for their chicks. Species that are unable to defend feeding territories, such as [[seabird]]s and [[Swift (bird)|swift]]s, often breed in [[Bird colony|colonies]] instead; this is thought to offer protection from predators. Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense.<ref>{{cite journal |last1=Kokko |first1=H |last2=Harris |first2=M |last3=Wanless |first3=S |year=2004 |title=Competition for breeding sites and site-dependent population regulation in a highly colonial seabird, the common guillemot ''Uria aalge'' |journal=Journal of Animal Ecology |volume=73 |issue=2| pages=367–376 |doi=10.1111/j.0021-8790.2004.00813.x|doi-access=free |bibcode=2004JAnEc..73..367K }}</ref>

All birds lay [[amniotic egg]]s with hard shells made mostly of [[calcium carbonate]].<ref name = "Gill"/> Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay [[camouflage]]d eggs. There are many exceptions to this pattern, however; the ground-nesting [[nightjar]]s have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of [[brood parasites]] have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts.<ref>{{cite journal | last1 = Booker | first1 = L | last2 = Booker | first2 = M | year = 1991 | title = Why Are Cuckoos Host Specific? | journal = [[Oikos (journal)|Oikos]] | volume = 57 | issue = 3| pages = 301–309 | doi = 10.2307/3565958 | jstor=3565958}}</ref>
[[File:Golden-backed Weaver.jpg|thumb|left|alt=Yellow weaver (bird) with black head hangs an upside-down nest woven out of grass fronds.|Male [[golden-backed weaver]]s construct elaborate suspended nests out of grass.]]
Bird eggs are usually laid in a [[Bird nest|nest]]. Most species create somewhat elaborate nests, which can be cups, domes, plates, mounds, or burrows.<ref name = "Hansell">{{cite book |last1=Hansell |first1=M |year=2000 |title=Bird Nests and Construction Behaviour |publisher=University of Cambridge Press |isbn=0-521-46038-7}}</ref> Some bird nests can be a simple scrape, with minimal or no lining; most seabird and wader nests are no more than a scrape on the ground. Most birds build nests in sheltered, hidden areas to avoid predation, but large or colonial birds—which are more capable of defence—may build more open nests. During nest construction, some species seek out plant matter from plants with parasite-reducing toxins to improve chick survival,<ref>{{cite journal | last1 = Lafuma | first1 = L. | last2 = Lambrechts | first2 = M. | last3 = Raymond | first3 = M. | year = 2001 | title = Aromatic plants in bird nests as a protection against blood-sucking flying insects? | journal = Behavioural Processes | volume = 56 | issue = 2| pages = 113–120 | doi = 10.1016/S0376-6357(01)00191-7 | pmid = 11672937 | bibcode = 2001BehPr..56..113L }}</ref> and feathers are often used for nest insulation.<ref name = "Hansell"/> Some bird species have no nests; the cliff-nesting [[common guillemot]] lays its eggs on bare rock, and male [[emperor penguin]]s keep eggs between their body and feet. The absence of nests is especially prevalent in open habitat ground-nesting species where any addition of nest material would make the nest more conspicuous. Many ground nesting birds lay a clutch of eggs that hatch synchronously, with [[precocial]] chicks led away from the nests ([[nidifugous]]) by their parents soon after hatching.<ref>{{cite book |author1=Collias, Nicholas E. |title=Nest building and bird behavior |author2=Collias, Elsie C. |publisher=Princeton University Press |year=1984 |isbn=0-691-08358-4 |place=Princeton, NJ |pages=16–17, 26}}</ref>

[[File:Eastern Phoebe-nest-Brown-headed-Cowbird-egg.jpg|thumb|alt= Nest made of straw with five white eggs and one grey speckled egg|Nest of an [[eastern phoebe]] that has been parasitised by a [[brown-headed cowbird]]]]
[[Egg incubation|Incubation]], which regulates temperature for chick development, usually begins after the last egg has been laid.<ref name = "Gill"/> In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Warmth from parents passes to the eggs through [[brood patch]]es, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process; adult albatrosses, for instance, lose as much as {{convert|83|g}} of body weight per day of incubation.<ref>{{cite book |last1=Warham |first1=J. |year=1990 |title=The Petrels: Their Ecology and Breeding Systems |location=London |publisher=[[Academic Press]] |isbn=0-12-735420-4}}</ref> The warmth for the incubation of the eggs of [[megapode]]s comes from the sun, decaying vegetation or volcanic sources.<ref>{{cite book |last1=Jones |first1=DN |last2=Dekker |first2=René WRJ |last3=Roselaar |first3=Cees S |year=1995 |chapter=The Megapodes |title=Bird Families of the World 3. |publisher=[[Oxford University Press]] |location=Oxford |isbn=0-19-854651-3}}</ref> Incubation periods range from 10 days (in [[woodpecker]]s, [[cuckoo]]s and [[passerine]] birds) to over 80 days (in albatrosses and [[Kiwi (bird)|kiwi]]s).<ref name = "Gill"/>

The diversity of characteristics of birds is great, sometimes even in closely related species. Several avian characteristics are compared in the table below.<ref name="AnAge">{{cite web|title=AnAge: The animal ageing and longevity database|url=http://genomics.senescence.info/species/|publisher=Human Ageing and Genomics Resources|access-date=26 September 2014}}</ref><ref name="ADW">{{cite web|title=Animal diversity web|url=http://animaldiversity.ummz.umich.edu/|publisher=University of Michigan, Museum of Zoology |access-date=26 September 2014}}</ref>

{| class="wikitable sortable"
|-
! Species
! Adult weight<br />(grams)
! Incubation<br />(days)
! Clutches<br />(per year)
! Clutch size
|-
| [[Ruby-throated hummingbird]] (''Archilochus colubris'')
| 3
| 13
| 2.0
| 2
|-
| [[House sparrow]] (''Passer domesticus'')
| 25
| 11
| 4.5
| 5
|-
| [[Greater roadrunner]] (''Geococcyx californianus'')
| 376
| 20
| 1.5
| 4
|-
| [[Turkey vulture]] (''Cathartes aura'')
| 2,200
| 39
| 1.0
| 2
|-
| [[Laysan albatross]] (''Phoebastria immutabilis'')
| 3,150
| 64
| 1.0
| 1
|-
| [[Magellanic penguin]] (''Spheniscus magellanicus'')
| 4,000
| 40
| 1.0
| 1
|-
| [[Golden eagle]] (''Aquila chrysaetos'')
| 4,800
| 40
| 1.0
| 2
|-
| [[Wild turkey]] (''Meleagris gallopavo'')
| 6,050
| 28
| 1.0
| 11
|}

====Parental care and fledging====
{{Main|Parental care in birds}}

At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed ''[[altricial]]'', and tend to be born small, [[Blindness|blind]], immobile and naked; chicks that are mobile and feathered upon hatching are termed ''[[precocial]]''. Altricial chicks need help [[thermoregulation|thermoregulating]] and must be brooded for longer than precocial chicks. The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on an "altricial-precocial spectrum".<ref name="Urfi2011">{{cite book|last=Urfi|first=A. J.|title=The Painted Stork: Ecology and Conservation|url=https://books.google.com/books?id=_9tczTapYXMC&pg=PA88|year=2011|publisher=Springer Science & Business Media|isbn=978-1-4419-8468-5|page=88}}</ref> Chicks at neither extreme but favouring one or the other may be termed {{Birdgloss|semi-precocial}}<ref name="Khanna2005">{{cite book|last=Khanna|first=D. R.|title=Biology of Birds|url=https://books.google.com/books?id=fDblIChi7KwC&pg=PA109|year=2005|publisher=Discovery Publishing House|isbn=978-81-7141-933-3|page=109}}</ref> or {{Birdgloss|semi-altricial}}.<ref name="Scott2008">{{cite book|last=Scott|first=Lynnette|title=Wildlife Rehabilitation|url=https://books.google.com/books?id=FpAOAQAAMAAJ|year=2008|publisher=National Wildlife Rehabilitators Association|isbn=978-1-931439-23-7|page=50}}</ref>
[[File:White-breasted Woodswallow chicks in nest.jpg|thumb|alt=Looking down on three helpless blind chicks in a nest within the hollow of a dead tree trunk|right|[[Altricial]] chicks of a [[white-breasted woodswallow]]]]

The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in [[megapode]]s ends at hatching; the newly hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately.<ref>{{cite book |last1=Elliot |first1=A |year=1994 |chapter=Family Megapodiidae (Megapodes) |title=Handbook of the Birds of the World |series=Vol. 2: New World Vultures to Guineafowl |editor-last1=del Hoyo |editor-first1=J. |editor-last2=Elliott |editor-first2=A. |editor-last3=Sargatal |editor-first3=J. |publisher=Lynx Edicions |location=Barcelona |isbn=84-87334-15-6 |title-link=Handbook of the Birds of the World }}</ref> At the other extreme, many seabirds have extended periods of parental care, the longest being that of the [[great frigatebird]], whose chicks take up to six months to [[fledge]] and are fed by the parents for up to an additional 14 months.<ref>{{cite book |last1=Metz |first1=V. G. |last2=Schreiber |first2=E. A. |year=2002|chapter=Great Frigatebird (''Fregata minor'') |title=The Birds of North America, No 681 |editor-last1=Poole |editor-first1=A. |editor-last2=Gill |editor-first2=F. |publisher=The Birds of North America Inc |location=Philadelphia}}</ref> The ''chick guard stage'' describes the period of breeding during which one of the adult birds is permanently present at the nest after chicks have hatched. The main purpose of the guard stage is to aid offspring to thermoregulate and protect them from predation.<ref>{{cite book |last1=Young |first1=Euan |title=Skua and Penguin. Predator and Prey |publisher=Cambridge University Press |year=1994 |page=453}}</ref>
[[File:Calliope-nest edit.jpg|thumb|alt=Hummingbird perched on edge of tiny nest places food into mouth of one of two chicks|left|A female [[calliope hummingbird]] feeding fully grown chicks]]
In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of only one sex. In some species, [[helpers at the nest|other members]] of the same species—usually close relatives of the [[breeding pair]], such as offspring from previous broods—will help with the raising of the young.<ref>{{cite journal | last1 = Ekman | first1 = J. | year = 2006 | title = Family living amongst birds | journal = [[Journal of Avian Biology]] | volume = 37 | issue = 4| pages = 289–298 | doi = 10.1111/j.2006.0908-8857.03666.x | bibcode = 2006JAvBi..37..289E }}</ref> Such [[alloparenting]] is particularly common among the [[Corvida]], which includes such birds as the true [[Corvidae|crows]], [[Australian magpie]] and [[fairy-wren]]s,<ref>{{Cite book|vauthors=Cockburn A |veditors=Floyd R, Sheppard A, de Barro P |title=Frontiers in Population Ecology|year=1996|publisher=CSIRO|location=Melbourne|pages=21–42|chapter=Why do so many Australian birds cooperate? Social evolution in the Corvida}}</ref> but has been observed in species as different as the [[Rifleman (bird)|rifleman]] and [[red kite]]. Among most groups of animals, [[Paternal care|male parental care]] is rare. In birds, however, it is quite common—more so than in any other vertebrate class.<ref name = "Gill"/> Although territory and nest site defence, incubation, and chick feeding are often shared tasks, there is sometimes a [[division of labour]] in which one mate undertakes all or most of a particular duty.<ref>{{Cite journal|last=Cockburn|first=Andrew|year=2006|title=Prevalence of different modes of parental care in birds |doi=10.1098/rspb.2005.3458|journal=[[Proceedings of the Royal Society B]]|volume=273|issue=1592|pages=1375–1383|pmid=16777726|pmc=1560291}}</ref>

The point at which chicks [[fledge]] varies dramatically. The chicks of the ''[[Synthliboramphus]]'' murrelets, like the [[ancient murrelet]], leave the nest the night after they hatch, following their parents out to sea, where they are raised away from terrestrial predators.<ref>{{cite book |last1=Gaston |first1=AJ |year=1994 |chapter=Ancient Murrelet (''Synthliboramphus antiquus'') |title=The Birds of North America, No. 132 |editor-first1=A. |editor-last1=Poole |editor-first2=F. |editor-last2=Gill |location=Philadelphia & Washington, D.C. |publisher=The Academy of Natural Sciences & The American Ornithologists' Union}}</ref> Some other species, such as ducks, move their chicks away from the nest at an early age. In most species, chicks leave the nest just before, or soon after, they are able to fly. The amount of parental care after fledging varies; albatross chicks leave the nest on their own and receive no further help, while other species continue some supplementary feeding after fledging.<ref>{{cite journal | last1 = Schaefer | first1 = H. C. | last2 = Eshiamwata | first2 = G. W. | last3 = Munyekenye | first3 = F. B. | last4 = Böhning-Gaese | first4 = K. | year = 2004 | title = Life-history of two African ''Sylvia'' warblers: low annual fecundity and long post-fledging care | journal = [[Ibis (journal)|Ibis]] | volume = 146 | issue = 3| pages = 427–437 | doi = 10.1111/j.1474-919X.2004.00276.x | bibcode = 2004Ibis..146..427S }}</ref> Chicks may also follow their parents during their first [[bird migration|migration]].<ref>{{cite journal | last1 = Alonso | first1 = J. C. | last2 = Bautista | first2 = L. M. | last3 = Alonso | first3 = J. A. | year = 2004 | title = Family-based territoriality vs flocking in wintering common cranes ''Grus grus'' | journal = [[Journal of Avian Biology]] | volume = 35 | issue = 5| pages = 434–444 | doi = 10.1111/j.0908-8857.2004.03290.x | hdl = 10261/43767 }}</ref>

====Brood parasites====
{{Main|Brood parasite}}

[[File:Reed warbler cuckoo.jpg|thumb|upright|right|alt=Small brown bird places an insect in the bill of much larger grey bird in nest|[[Reed warbler]] raising a [[common cuckoo]], a [[brood parasite]]]]
[[Brood parasitism]], in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism.<ref name="brood">{{cite book |last1=Davies |first1=N. |year=2000 |title=Cuckoos, Cowbirds and other Cheats |publisher=[[T. & A. D. Poyser]] |location=London |isbn=0-85661-135-2}}</ref> After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either ''obligate brood parasites'', which must lay their eggs in the nests of other species because they are incapable of raising their own young, or ''non-obligate brood parasites'', which sometimes lay eggs in the nests of [[conspecific]]s to increase their reproductive output even though they could have raised their own young.<ref>{{cite journal |doi=10.1093/beheco/8.2.153 |last1=Sorenson |first1=M. |year=1997 |title=Effects of intra- and interspecific brood parasitism on a precocial host, the canvasback, ''Aythya valisineria'' |journal=Behavioral Ecology |volume=8 |issue=2| pages=153–161 |doi-access=free}}</ref> One hundred bird species, including [[honeyguide]]s, [[icterid]]s, and [[Black-headed duck|ducks]], are obligate parasites, though the most famous are the [[cuckoo]]s.<ref name="brood"/> Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks; this ensures that all food brought to the nest will be fed to the parasitic chicks.<ref>{{cite journal| last1=Spottiswoode| first1=C. N.| last2=Colebrook-Robjent| first2=J. F. R.| title=Egg puncturing by the brood parasitic Greater Honeyguide and potential host counteradaptations| journal=Behavioral Ecology| volume=18| pages=792–799| year=2007| doi=10.1093/beheco/arm025| issue=4| doi-access=free| hdl=10.1093/beheco/arm025| hdl-access=free}}</ref>

====Sexual selection====
[[File:Peacock Flying.jpg|thumb|upright=1.35|right|The peacock tail in flight, the classic example of a [[Fisherian runaway]]]]
{{Main|Sexual selection in birds}}

Birds have [[evolution|evolved]] a variety of [[mating]] behaviours, with the [[peafowl|peacock]] tail being perhaps the most famous example of [[sexual selection]] and the [[Fisherian runaway]]. Commonly occurring [[sexual dimorphism]]s such as size and colour differences are energetically costly attributes that signal competitive breeding situations.<ref name=edwards2012>{{cite journal|last=Edwards|first=DB|title=Immune investment is explained by sexual selection and pace-of-life, but not longevity in parrots (Psittaciformes).|journal=PLOS ONE|year=2012|volume=7|issue=12|article-number=e53066|pmid=23300862|doi=10.1371/journal.pone.0053066|pmc=3531452|bibcode=2012PLoSO...753066E|doi-access=free}}</ref> Many types of avian [[sexual selection]] have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signalling adaptations ensure that sexually selected ornaments such as plumage colouration and [[courtship behavior|courtship behaviour]] are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours.<ref name=doutrelant2012>{{cite journal|last=Doutrelant|first=C|author2=Grégoire, A |author3=Midamegbe, A |author4=Lambrechts, M |author5= Perret, P |title=Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits.|journal=[[Journal of Animal Ecology]]|date=January 2012|volume=81|issue=1|pages=87–96|pmid=21819397|doi=10.1111/j.1365-2656.2011.01889.x|doi-access=free|bibcode=2012JAnEc..81...87D}}</ref>

====Inbreeding depression====
{{Main|Inbreeding depression}}

[[Inbreeding]] causes early death ([[inbreeding depression]]) in the [[zebra finch]] ''Taeniopygia guttata''.<ref name="pmid22643890">{{cite journal |vauthors=Hemmings NL, Slate J, Birkhead TR |title=Inbreeding causes early death in a passerine bird |journal=Nat Commun |volume=3 |article-number=863 |year=2012 |pmid=22643890 |doi=10.1038/ncomms1870 |bibcode=2012NatCo...3..863H |doi-access=free }}</ref> Embryo survival (that is, hatching success of fertile eggs) was significantly lower for [[sibling|sib-sib]] mating pairs than for unrelated pairs.<ref>{{Cite journal |last1=Hemmings |first1=N. L. |last2=Slate |first2=J. |last3=Birkhead |first3=T. R. |date=29 May 2012 |title=Inbreeding causes early death in a passerine bird |journal=Nature Communications |language=en |volume=3 |issue=1 |page=863 |doi=10.1038/ncomms1870 |pmid=22643890 |bibcode=2012NatCo...3..863H |doi-access=free }}</ref>

[[Darwin's finches|Darwin's finch]] ''Geospiza scandens'' experiences [[inbreeding depression]] (reduced survival of offspring) and the magnitude of this effect is influenced by environmental conditions such as low food availability.<ref name="pmid12144022">{{cite journal |vauthors=Keller LF, Grant PR, Grant BR, Petren K |title=Environmental conditions affect the magnitude of inbreeding depression in survival of Darwin's finches |journal=Evolution |volume=56 |issue=6 |pages=1229–1239 |year=2002 |pmid=12144022 |doi=10.1111/j.0014-3820.2002.tb01434.x }}</ref>

====Inbreeding avoidance====
{{Main|Inbreeding avoidance}}

Incestuous matings by the [[Purple-crowned fairywren|purple-crowned fairy wren]] ''Malurus coronatus'' result in severe fitness costs due to [[inbreeding depression]] (greater than 30% reduction in hatchability of eggs).<ref name=Kingma>{{cite journal | last1 = Kingma | first1 = SA | last2 = Hall | first2 = ML | last3 = Peters | first3 = A | year = 2013 | title = Breeding synchronization facilitates extrapair mating for inbreeding avoidance | journal = Behavioral Ecology | volume = 24 | issue = 6| pages = 1390–1397 | doi = 10.1093/beheco/art078 | doi-access = free | hdl = 10.1093/beheco/art078 | hdl-access = free }}</ref> Females paired with related males may undertake extra pair matings (see [[Promiscuity#Other animals]] for 90% frequency in avian species) that can reduce the negative effects of inbreeding. However, there are ecological and demographic constraints on extra pair matings. Nevertheless, 43% of broods produced by incestuously paired females contained extra pair young.<ref name=Kingma />

Inbreeding depression occurs in the [[great tit]] (''Parus major'') when the offspring produced as a result of a mating between close relatives show reduced fitness. In natural populations of ''Parus major'', inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative.<ref name="pmid18211876">{{cite journal |vauthors=Szulkin M, Sheldon BC |title=Dispersal as a means of inbreeding avoidance in a wild bird population |journal=Proc. Biol. Sci. |volume=275 |issue=1635 |pages=703–711 |year=2008 |pmid=18211876 |pmc=2596843 |doi=10.1098/rspb.2007.0989 }}</ref>

[[Southern pied babbler]]s ''Turdoides bicolor'' appear to avoid inbreeding in two ways. The first is through dispersal, and the second is by avoiding familiar group members as mates.<ref name="pmid22471769">{{cite journal |vauthors=Nelson-Flower MJ, Hockey PA, O'Ryan C, Ridley AR |title=Inbreeding avoidance mechanisms: dispersal dynamics in cooperatively breeding southern pied babblers |journal=J Anim Ecol |volume=81 |issue=4 |pages=876–883 |year=2012 |pmid=22471769 |doi=10.1111/j.1365-2656.2012.01983.x |doi-access=free |bibcode=2012JAnEc..81..876N }}</ref>

[[Cooperative breeding]] in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin.<ref name="pmid26577076">{{cite journal |vauthors=Riehl C, Stern CA |title=How cooperatively breeding birds identify relatives and avoid incest: New insights into dispersal and kin recognition |journal=BioEssays |volume=37 |issue=12 |pages=1303–1308 |year=2015 |pmid=26577076 |doi=10.1002/bies.201500120 }}</ref> Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups. In general, inbreeding is avoided because it leads to a reduction in progeny fitness ([[inbreeding depression]]) due largely to the homozygous expression of deleterious recessive alleles.<ref name="pmid19834483">{{cite journal |vauthors=Charlesworth D, Willis JH |title=The genetics of inbreeding depression |journal=Nat. Rev. Genet. |volume=10 |issue=11 |pages=783–796 |year=2009 |pmid=19834483 |doi=10.1038/nrg2664 }}</ref> [[Outcrossing|Cross-fertilisation]] between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.<ref name="pmid3324702">{{cite book |vauthors=Bernstein H, Hopf FA, Michod RE |title=Molecular Genetics of Development |chapter=The Molecular Basis of the Evolution of Sex |volume=24 |pages=323–370 |year=1987 |pmid=3324702 |doi= 10.1016/s0065-2660(08)60012-7|series=Advances in Genetics |isbn=978-0-12-017624-3 }}</ref><ref>{{cite book |last1=Michod |first1=R.E. |year=1994 |title=Eros and Evolution: A Natural Philosophy of Sex |publisher=Addison-Wesley Publishing Company |location=Reading, Massachusetts |isbn=978-0-201-44232-8}}</ref>

==Ecology==
[[File:Pinzón azul de Gran Canaria (macho), M. A. Peña.jpg|thumb|right|[[Gran Canaria blue chaffinch]], an example of a bird highly specialised in its habitat, in this case in the [[Pinus canariensis|Canarian pine]] forests]]
Birds occupy a wide range of ecological positions.<ref name = "flycatcher"/> While some birds are generalists, others are highly specialised in their habitat or food requirements. Even within a single habitat, such as a forest, the [[Ecological niche|niches]] occupied by different species of birds vary, with some species feeding in the [[forest canopy]], others beneath the canopy, and still others on the forest floor. Forest birds may be [[insectivore]]s, [[frugivore]]s, or [[nectarivore]]s. Aquatic birds generally feed by fishing, plant eating, and piracy or [[kleptoparasitism]]. Many grassland birds are granivores. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised [[scavenger]]s. Birds are also preyed upon by a range of mammals including a few [[Avivore|avivorous]] bats.<ref>{{Cite journal |last1=Gong |first1=Lixin |last2=Shi |first2=Biye |last3=Wu |first3=Hui |last4=Feng |first4=Jiang |last5=Jiang |first5=Tinglei |date=2021 |title=Who's for dinner? Bird prey diversity and choice in the great evening bat, Ia io |journal=Ecology and Evolution |volume=11 |issue=13 |pages=8400–8409 |doi=10.1002/ece3.7667 |pmc=8258197 |pmid=34257905|bibcode=2021EcoEv..11.8400G }}</ref> A wide range of endo- and ectoparasites depend on birds and some parasites that are transmitted from parent to young have [[Coevolution|co-evolved]] and show host-specificity.<ref>{{Cite journal |last1=Križanauskienė |first1=Asta |last2=Hellgren |first2=Olof |last3=Kosarev |first3=Vladislav |last4=Sokolov |first4=Leonid |last5=Bensch |first5=Staffan |last6=Valkiūnas |first6=Gediminas |date=2006 |title=Variation in host specificity between species of avian hemosporidian parasites: evidence from parasite morphology and cytochrome b gene sequences |journal=Journal of Parasitology |volume=92 |issue=6 |pages=1319–1324 |doi=10.1645/GE-873R.1 |pmid=17304814 |bibcode=2006JPara..92.1319K }}</ref>

Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal.<ref name = "Clout">{{cite journal | last1 = Clout | first1 = M | last2 = Hay | first2 = J | year = 1989 | title = The importance of birds as browsers, pollinators and seed dispersers in New Zealand forests | url = http://www.newzealandecology.org/nzje/free_issues/NZJEcol12_s_27.pdf | journal = New Zealand Journal of Ecology | volume = 12 | pages = 27–33 }}</ref> Plants and pollinating birds often [[coevolution|coevolve]],<ref>{{cite journal | last1=Stiles| first1=F. Gary| title=Geographical Aspects of Bird-Flower Coevolution, with Particular Reference to Central America | journal=Annals of the Missouri Botanical Garden | volume=68 | issue=2 | pages=323–351 | year=1981 |doi= 10.2307/2398801 | jstor=2398801 | bibcode=1981AnMBG..68..323S| url=https://www.biodiversitylibrary.org/part/38387 }}</ref> and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.<ref>{{cite journal |last1=Temeles |first1=Ethan J. |last2=Linhart |first2=Yan B. |last3=Masonjones |first3=Michael |last4=Masonjones |first4=Heather D. |title=The Role of Flower Width in Hummingbird Bill Length-Flower Length Relationships 1 |journal=Biotropica |date=March 2002 |volume=34 |issue=1 |pages=68–80 |doi=10.1111/j.1744-7429.2002.tb00243.x |bibcode=2002Biotr..34...68T }}</ref>

Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfil ecological roles typically played by larger animals. For example, in New Zealand nine species of [[moa]] were important browsers, as are the [[kererū]] and [[kōkako]] today.<ref name = "Clout"/> Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.<ref>{{cite journal | last1=Bond | first1=William J. | last2=Lee | first2=William G. | last3=Craine | first3=Joseph M. | title=Plant structural defences against browsing birds: a legacy of New Zealand's extinct moas | journal=Oikos | volume=104 | pages=500–508 | year=2004 | doi = 10.1111/j.0030-1299.2004.12720.x | issue=3| bibcode=2004Oikos.104..500B }}</ref>

Many birds act as [[ecosystem engineer]]s through the construction of nests, which provide important microhabitats and food for hundreds of species of invertebrates.<ref>{{Cite journal|last1=Berner|first1=Lewis|last2=Hicks|first2=Ellis A.|date=June 1959|title=Checklist and Bibliography on the Occurrence of Insects in Birds Nests|journal=The Florida Entomologist|volume=42|issue=2|page=92|doi=10.2307/3492142|jstor=3492142 |url=http://archive.org/details/checklistbibliog00hick |doi-access=free}}</ref><ref>{{Cite journal|last1=Boyes|first1=Douglas H.|last2=Lewis|first2=Owen T.|date=2019|title=Ecology of Lepidoptera associated with bird nests in mid-Wales, UK|journal=Ecological Entomology |volume=44|issue=1|pages=1–10|doi=10.1111/een.12669 |doi-access=free|bibcode=2019EcoEn..44....1B }}</ref> Nesting [[seabird]]s may affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of [[guano]], which may enrich the local soil<ref>{{cite journal | last1 = Wainright | first1 = S. | last2 = Haney | first2 = J. | last3 = Kerr | first3 = C. | last4 = Golovkin | first4 = A. | last5 = Flint | first5 = M. | year = 1998 | title = Utilization of nitrogen derived from seabird guano by terrestrial and marine plants at St. Paul, Pribilof Islands, Bering Sea, Alaska | journal = Marine Biology| volume = 131 | issue = 1| pages = 63–71 | doi=10.1007/s002270050297| bibcode = 1998MarBi.131...63W }}</ref> and the surrounding seas.<ref>{{cite journal | doi = 10.3354/meps032247 | last1 = Bosman | first1 = A. | last2 = Hockey | first2 = A. | year = 1986 | title = Seabird guano as a determinant of rocky intertidal community structure | journal = Marine Ecology Progress Series | volume = 32 |pages = 247–257 | bibcode = 1986MEPS...32..247B | doi-access = free }}</ref>

A wide variety of [[avian ecology field methods]], including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.<ref>{{cite book |author2=Newton, Ian |author1=Sutherland, William J. |title=Bird Ecology and Conservation. A Handbook of Techniques |author3=Green, Rhys E. |year=2004 |publisher=Oxford University Press |isbn=0-19-852085-9}}</ref>

==Relationship with humans==
{{Main|Human uses of birds}}

[[File:Industrial-Chicken-Coop.JPG|thumb|alt=Two rows of cages in a dark barn with many white chickens in each cage|[[Industrial farming]] of [[chicken]]s]]

Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man.<ref>{{Cite book|last1=Bonney|first1=Rick | last2=Rohrbaugh| first2=Ronald Jr.|title=Handbook of Bird Biology| place=Princeton, NJ|publisher=Princeton University Press|year=2004| edition=Second|isbn=0-938027-62-X}}</ref> Sometimes, these relationships are [[Mutualism (biology)|mutualistic]], like the cooperative honey-gathering among [[honeyguide]]s and African peoples such as the [[Borana people|Borana]].<ref>{{Cite journal |doi = 10.1111/j.1523-1739.1990.tb00272.x|title = The Fallacy, Fact, and Fate of Guiding Behavior in the Greater Honeyguide|journal = Conservation Biology|volume = 4|pages = 99–101|year = 1990|last1 = Dean|first1 = W. R. J.|last2 = Siegfried|first2 = W. ROY|last3 = MacDonald|first3 = I. A. W.| issue=1 | bibcode=1990ConBi...4...99D }}</ref> Other times, they may be [[Commensalism|commensal]], as when species such as the [[house sparrow]]<ref>{{cite journal | last1=Singer | first1=R. | last2=Yom-Tov | first2=Y. | title=The Breeding Biology of the House Sparrow Passer domesticus in Israel | journal=Ornis Scandinavica | volume=19 | issue=2 | pages=139–144 | year=1988 |doi=10.2307/3676463 | jstor=3676463}}</ref> have benefited from human activities. Several species have reconciled to habits of farmers who practice traditional farming. Examples include the [[Sarus crane|Sarus Crane]] that begins nesting in India when farmers flood the fields in anticipation of rains,<ref>{{Cite journal |last=Sundar |first=K. S. Gopi |date=2009 |title=Are rice paddies suboptimal breeding habitat for Sarus Cranes in Uttar Pradesh, India? |journal=The Condor |volume=111 |issue=4 |pages=611–623|doi=10.1525/cond.2009.080032 }}</ref> and the [[Asian woolly-necked stork|woolly-necked storks]] that have taken to nesting on a short tree grown for agroforestry beside fields and canals.<ref>{{Cite journal |last1=Kittur |first1=Swati |last2=Sundar |first2=K. S. Gopi |date=2021 |title=Of irrigation canals and multifunctional agroforestry: Traditional agriculture facilitates Woolly-necked Stork breeding in a north Indian agricultural landscape |journal=Global Ecology and Conservation |volume=30 |article-number=e01793|doi=10.1016/j.gecco.2021.e01793 |doi-access=free |bibcode=2021GEcoC..3001793K }}</ref> Several bird species have become commercially significant agricultural pests,<ref>{{Cite journal | doi=10.1111/j.1474-919X.1990.tb01048.x | author=Dolbeer, Richard | year=1990 | title=Ornithology and integrated pest management: Red-winged blackbirds ''Agleaius phoeniceus'' and corn | journal=[[Ibis (journal)|Ibis]] | volume=132 | issue=2| pages=309–322 | bibcode=1990Ibis..132..309D | url=https://zenodo.org/record/1230653}}</ref> and some pose an [[bird strike|aviation hazard]].<ref>{{Cite journal | last1=Dolbeer | first1=R. | last2=Belant | first2=J. | last3=Sillings | first3=J. | year=1993 | title=Shooting Gulls Reduces Strikes with Aircraft at John F. Kennedy International Airport | journal=Wildlife Society Bulletin | volume=21 | pages=442–450 }}</ref> Human activities can also be detrimental, and have threatened numerous bird species with extinction ([[hunting]], [[avian lead poisoning]], [[pesticide]]s, [[roadkill]], [[wind turbine]] kills<ref>{{cite web |url=http://www.audubon.org/news/will-wind-turbines-ever-be-safe-birds |title=Will Wind Turbines Ever Be Safe for Birds? |first1=Emma |last1=Bryce |website=Audubon |location=US |publisher=[[National Audubon Society]] |date=16 March 2016 |access-date=19 March 2017}}</ref> and predation by pet [[cat]]s and [[dog]]s are common causes of death for birds).<ref>{{cite news |last1=Zimmer |first1=Carl |title=Birds Are Vanishing From North America |url=https://www.nytimes.com/2019/09/19/science/bird-populations-america-canada.html?campaign_id=60&instance_id=0&segment_id=17168&user_id=579ae23cfcbd75c9aac87cb571cc201c&regi_id=72995439ing-news |access-date=19 September 2019 |work=The New York Times |date=19 September 2019}}</ref>

Birds can act as vectors for spreading diseases such as [[psittacosis]], [[salmonellosis]], [[campylobacteriosis]], mycobacteriosis (avian [[tuberculosis]]), [[avian influenza]] (bird flu), [[giardiasis]], and [[cryptosporidiosis]] over long distances. Some of these are [[zoonosis|zoonotic diseases]] that can also be transmitted to humans.<ref>{{cite journal | doi=10.3121/cmr.1.1.5 | last1=Reed | first1=K. D. | last2=Meece | first2=J. K. | last3=Henkel | first3=J. S. | last4=Shukla | first4=S. K. | title=Birds, Migration and Emerging Zoonoses: West Nile Virus, Lyme Disease, Influenza A and Enteropathogens | journal=Clinical Medicine & Research | volume=1 | issue=1 | pages=5–12 | year=2003 | pmid=15931279 | pmc=1069015 }}</ref>

===Economic importance===
{{See also|Pet#Birds}}
[[File:FishingCormorants.jpg|thumb|left|upright|alt=Illustration of fisherman on raft with pole for punting and numerous black birds on raft|The use of cormorants by Asian fishermen is in steep decline but survives in some areas as a tourist attraction.]]

Domesticated birds raised for meat and eggs, called [[poultry]], are the largest source of animal protein eaten by humans; in 2003, {{Nowrap|76 million}} tons of poultry and {{Nowrap|61 million}} tons of eggs were produced worldwide.<ref>{{cite book |last1=Brown|first1=Lester|title=Outgrowing the Earth: The Food Security Challenge in an Age of Falling Water Tables and Rising Temperatures |chapter-url=http://www.earth-policy.org/datacenter/xls/book_ote_ch3_2.xls|year=2005|publisher=earthscan |isbn=978-1-84407-185-2|chapter=3: Moving Up the Food Chain Efficiently. }}</ref> [[Chicken]]s account for much of human poultry consumption, though domesticated [[domesticated turkey|turkeys]], [[domestic duck|ducks]], and [[domestic goose|geese]] are also relatively common.<ref>{{cite web |title=Poultry species: Gateway to poultry production and products |url=https://www.fao.org/poultry-production-products/production/poultry-species/en/ |website=Food and Agriculture Organization of the United Nations |publisher=FAO |access-date=27 January 2023}}</ref> Many species of birds are also hunted for meat. Bird hunting is primarily a recreational activity except in extremely undeveloped areas. The most important birds hunted in North and South America are waterfowl; other widely hunted birds include [[pheasant]]s, [[wild turkey]]s, quail, [[dove]]s, [[partridge]], [[grouse]], [[snipe]], and [[woodcock]].<ref>{{cite book |doi=10.5962/bhl.title.127802 |title=General and particular descriptions of the vertebrated animals: Arranged conformably to the modern discoveries and improvements in zoology. Order Carnivora |date=1821 |last1=Griffith |first1=Edward |last2=Wood |first2=W. |last3=Davison |first3=Thomas }}{{page needed|date=January 2025}}</ref> [[Muttonbirding]] is also popular in Australia and New Zealand.<ref>{{cite journal | last1=Hamilton | first1=S. | year=2000 | title=How precise and accurate are data obtained using. an infrared scope on burrow-nesting sooty shearwaters ''Puffinus griseus''? | url=http://www.marineornithology.org/PDF/28_1/28_1_1.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.marineornithology.org/PDF/28_1/28_1_1.pdf |archive-date=9 October 2022 |url-status=live | journal=Marine Ornithology | volume=28 | issue=1| pages=1–6 | doi=10.5038/2074-1235.28.1.450 }}</ref> Although some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species.<ref>{{cite journal | last1=Keane | first1=Aidan | last2=Brooke | first2=M. de L. | last3=McGowan | first3=P. J. K. | title=Correlates of extinction risk and hunting pressure in gamebirds (Galliformes) | journal=Biological Conservation | volume=126 | pages=216–233 | year=2005 |doi=10.1016/j.biocon.2005.05.011 | issue=2| bibcode=2005BCons.126..216K }}</ref>

Other commercially valuable products from birds include feathers (especially the [[Down feather|down]] of geese and ducks), which are used as insulation in clothing and bedding, and seabird faeces ([[guano]]), which is a valuable source of phosphorus and nitrogen. The [[War of the Pacific]], sometimes called the Guano War, was fought in part over the control of guano deposits.<ref>{{cite web|url=http://www.zum.de/whkmla/military/19cen/guanowar.html|title=The Guano War of 1865–1866|publisher=World History at KMLA|access-date=18 December 2007}}</ref>

Birds have been domesticated by humans both as pets and for practical purposes. Colourful birds, such as [[Parrot (family)|parrots]] and [[myna]]s, are bred in [[aviculture|captivity]] or kept as pets, a practice that has led to the illegal trafficking of some [[endangered species]].<ref>{{cite journal | doi=10.1017/S0030605306000056 | last1=Cooney | first1=R. | last2=Jepson | first2=P. | year=2006 | title=The international wild bird trade: what's wrong with blanket bans? | journal=Oryx | volume=40 | issue=1| pages=18–23 | bibcode=2006Oryx...40...18C | doi-access=free }}</ref> [[Falcon]]s and [[cormorant]]s have long been used for [[Falconry|hunting]] and [[Cormorant fishing|fishing]], respectively. [[Messenger pigeon]]s, used since at least 1&nbsp;AD, remained important as recently as [[World War II]]. Today, such activities are more common either as hobbies, for entertainment and tourism.<ref>{{Cite journal | doi=10.2307/4140937 | last1=Manzi | first1=M. | last2=Coomes | year=2002 | first2=O. T. | title=Cormorant fishing in Southwestern China: a Traditional Fishery under Siege. (Geographical Field Note) | journal=Geographical Review | volume=92 | issue=4| pages=597–603 | jstor=4140937 }}</ref>

Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, [[birding|birders]]) number in the millions.<ref>{{cite book |last1=Pullis La Rouche |first1=G. |year=2006 |chapter=Birding in the United States: a demographic and economic analysis |title=Waterbirds around the world |editor-first1=G. C. |editor-last1=Boere |editor-first2=C. A. |editor-last2=Galbraith |editor-first3=D. A. |editor-last3=Stroud |publisher=[[The Stationery Office]] |location=Edinburgh |pages=841–846 |url=http://www.jncc.gov.uk/PDF/pub07_waterbirds_part6.2.5.pdf |archive-url=https://web.archive.org/web/20110304124708/http://www.jncc.gov.uk/PDF/pub07_waterbirds_part6.2.5.pdf |archive-date=4 March 2011}}</ref> Many homeowners erect [[bird feeder]]s near their homes to attract various species. [[Bird feeding]] has grown into a multimillion-dollar industry; for example, an estimated 75% of households in Britain provide food for birds at some point during the winter.<ref>{{cite journal | doi=10.1111/j.1474-919x.2005.00430.x | last1=Chamberlain | first1=D. E. | last2=Vickery | first2=J. A. | last3=Glue | first3=D. E. | last4=Robinson | first4=R. A. | last5=Conway | first5=G. J. | last6=Woodburn | first6=R. J. W. | last7=Cannon | first7=A. R. | year=2005 | title=Annual and seasonal trends in the use of garden feeders by birds in winter | journal=[[Ibis (journal)|Ibis]] | volume=147 | issue=3| pages=563–575 | bibcode=2005Ibis..147..563C }}</ref>

===In religion and mythology===
[[File:Vogel Drei (Meister der Spielkarten).jpg|thumb|upright|alt=Woodcut of three long-legged and long-necked birds|The ''3 of Birds'' by the [[Master of the Playing Cards]], 15th-century Germany]]

Birds play prominent and diverse roles in religion and mythology.
In religion, birds may serve as either messengers or priests and leaders for a [[deity]], such as in the Cult of [[Makemake (mythology)|Makemake]], in which the [[Tangata manu]] of [[Easter Island]] served as chiefs<ref>{{Cite journal | last1=Routledge | first1=S. | last2=Routledge | first2=K. | year=1917 | title=The Bird Cult of Easter Island | journal=Folklore | volume=28 | issue=4| pages=337–355 | doi=10.1080/0015587X.1917.9719006 }}</ref> or as attendants, as in the case of [[Hugin and Munin]], the two [[common raven]]s who whispered news into the ears of the [[Norse god]] [[Odin]]. In several civilisations of [[History of Italy|ancient Italy]], particularly [[Etruscan mythology|Etruscan]] and [[Ancient Rome|Roman]] [[Religion in Ancient Rome|religion]], priests were involved in [[augur]]y, or interpreting the words of birds while the "auspex" (from which the word "auspicious" is derived) watched their activities to foretell events.<ref>{{cite web|last1=Ingersoll |first1=Ernest |year=1923 |url=https://archive.org/details/birdsinlegendfab00inge |via=Wayback Machine |title=Birds in legend, fable and folklore |publisher=Longmans, Green and Co. |page=214}}</ref>

They may also serve as [[Religious symbolism|religious symbols]], as when [[Jonah]] ({{Langx|he|יונה}}, [[dove]]) embodied the fright, passivity, mourning, and beauty traditionally associated with doves.<ref>{{cite journal| last1=Hauser| first1=A. J.| title=Jonah: In Pursuit of the Dove| journal=Journal of Biblical Literature| volume=104| issue=1| pages=21–37| year=1985 |doi=10.2307/3260591| jstor=3260591}}</ref> Birds have themselves been deified, as in the case of the [[common peacock]], which is perceived as Mother Earth by the people of southern India.<ref>{{cite journal | last1=Thankappan Nair | first1=P. | title=The Peacock Cult in Asia | journal=Asian Folklore Studies | volume=33 | issue=2 | pages=93–170 | year=1974 | doi=10.2307/1177550 | jstor=1177550}}</ref> In the ancient world, doves were used as symbols of the [[Mesopotamian goddess]] [[Inanna]] (later known as Ishtar),<ref name="Botterweck">{{cite book|last1=Botterweck|first1=G. Johannes|last2=Ringgren|first2=Helmer|date=1990|title=Theological Dictionary of the Old Testament|url=https://books.google.com/books?id=MCOd-uAEQy0C&q=Ishtar%20doves&pg=PA35|volume=VI|location=Grand Rapids, Michigan|publisher=Wm. B. Eerdmans Publishing Co.|isbn=0-8028-2330-0|pages=35–36}}</ref><ref name="Lewis">{{cite book|last1=Lewis|first1=Sian|last2=Llewellyn-Jones|first2=Lloyd|date=2018|title=The Culture of Animals in Antiquity: A Sourcebook with Commentaries|url=https://books.google.com/books?id=GvJFDwAAQBAJ&q=Ishtar%20doves&pg=PT335|location=New York City, New York and London, England|publisher=Routledge|isbn=978-1-315-20160-3|page=335}}</ref> the [[Ancient Canaanite religion|Canaanite]] mother goddess [[Asherah]],<ref name="Botterweck"/><ref name="Lewis"/><ref name="BAR">{{cite web |first1=Dorothy D. |last1=Resig |url=http://www.bib-arch.org/e-features/enduring-doves.asp |title=The Enduring Symbolism of Doves, From Ancient Icon to Biblical Mainstay |archive-url=https://web.archive.org/web/20130131120659/http://www.bib-arch.org/e-features/enduring-doves.asp |archive-date=31 January 2013 |website=BAR Magazine Bib-arch.org |date=9 February 2013 |access-date=5 March 2013}}</ref> and the Greek goddess [[Aphrodite]].<ref name="Botterweck"/><ref name="Lewis"/><ref name="Cyrino2010">{{cite book|last=Cyrino|first=Monica S.|date=2010|title=Aphrodite|series=Gods and Heroes of the Ancient World|url=https://books.google.com/books?id=7gyVn5GjXPkC&q=Aphrodite%20Monica%20S.%20Cyrino|location=New York City, New York and London, England|publisher=Routledge|isbn=978-0-415-77523-6|pages=120–123}}</ref><ref>{{cite book|last=Tinkle|first=Theresa|date=1996|title=Medieval Venuses and Cupids: Sexuality, Hermeneutics, and English Poetry|url=https://books.google.com/books?id=gjhLACGml2cC&q=Mary&pg=PA80|location=Stanford, California|publisher=Stanford University Press|isbn=978-0-8047-2515-6|page=81}}</ref><ref>{{cite book|last=Simon|first=Erika|date=1983|title=Festivals of Attica: An Archaeological Companion|url=https://books.google.com/books?id=IHFYM-IGCO8C&q=Aphrodisia%20festival&pg=PA48|location=Madison, WI|publisher=University of Wisconsin Press|isbn=0-299-09184-8}}</ref> In [[ancient Greece]], [[Athena]], the goddess of wisdom and patron deity of the city of [[Athens]], had a [[little owl]] as [[Owl of Athena|her symbol]].<ref name="Deacy, Susan 2001">{{cite book|last1=Deacy|first1=Susan|last2=Villing|first2=Alexandra|date=2001|title=Athena in the Classical World|publisher=Koninklijke Brill NV|location=Leiden, The Netherlands|isbn=978-90-04-12142-3}}</ref><ref name="Deacy2008">{{cite book|last=Deacy|first=Susan|date=2008|title=Athena|location=London and New York City|publisher=Routledge|isbn=978-0-415-30066-7|url=https://books.google.com/books?id=kIiCAgAAQBAJ&q=Athena%20and%20Ares%20Darmon&pg=PA163|pages=34–37, 74–75}}</ref><ref name="Nilsson1950">{{cite book|last=Nilsson|first=Martin Persson|author-link=Martin P. Nilsson|title=The Minoan-Mycenaean Religion and Its Survival in Greek Religion|location=New York City, New York|publisher=Biblo & Tannen|year=1950|isbn=0-8196-0273-6|edition=second|pages=491–496}}</ref> In religious images preserved from the Inca and Tiwanaku empires, birds are depicted in the process of transgressing boundaries between earthly and underground spiritual realms.<ref name="Smith2011Tiwanaku">{{cite journal |author=Smith, S. |year=2011 |title=Generative landscapes: the step mountain motif in Tiwanaku iconography. |journal=Ancient America |volume=12 |pages=1–69 |url=http://www.precolumbia.com/bearc/CAAS/AA12.pdf |format=Automatic PDF download |access-date=24 April 2014 |archive-date=6 January 2019 |archive-url=https://web.archive.org/web/20190106203516/http://www.precolumbia.com/bearc/CAAS/AA12.pdf }}</ref> Indigenous peoples of the central Andes maintain legends of birds passing to and from metaphysical worlds.<ref name="Smith2011Tiwanaku"/>

===In culture and folklore===
{{Further|Birds in culture}}
[[File:17.6-24-1974-Cuerda-seca-flisepanel.jpg|thumb|left|Painted [[tiles]] with design of birds from [[Qajar dynasty]], [[Iran]]]]

Birds have featured in culture and art since prehistoric times, when they were represented in early [[cave painting]]<ref>{{cite journal | last1=Meighan | first1=C.W. | title=Prehistoric Rock Paintings in Baja California | journal=American Antiquity | volume=31 | issue=3 | pages=372–392 | year=1966 |doi=10.2307/2694739 | jstor=2694739 }}</ref> and carvings.<ref>{{Cite journal |last=Conard |first=Nicholas J. |date=2003 |title=Palaeolithic ivory sculptures from southwestern Germany and the origins of figurative art |journal=Nature |language=en |volume=426 |issue=6968 |pages=830–832 |doi=10.1038/nature02186 |pmid=14685236 |bibcode=2003Natur.426..830C }}</ref> Some birds have been perceived as monsters, including the mythological [[Roc (mythology)|Roc]] and the [[Māori people|Māori]]'s legendary {{lang|mi|[[Poukai|Pouākai]]}}, a giant bird capable of snatching humans.<ref>{{cite book |last1=Tennyson |first1=A |last2=Martinson |first2=P |year=2006 |title=Extinct Birds of New Zealand |publisher=Te Papa Press |location=Wellington |isbn=978-0-909010-21-8}}</ref> Birds were later used as symbols of power, as in the magnificent [[Peacock Throne]] of the [[Mughal era|Mughal]] and [[History of Iran|Persian]] emperors.<ref>{{cite journal | last1=Clarke | first1=CP | year=1908 | title=A Pedestal of the Platform of the Peacock Throne | journal=The Metropolitan Museum of Art Bulletin | volume=3 | issue=10| pages=182–183 | doi=10.2307/3252550 | jstor=3252550}}</ref> With the advent of scientific interest in birds, many paintings of birds were commissioned for books.<ref>{{Cite web |title=Birds of Mughal India – Shangri La |url=https://www.shangrilahawaii.org/visit/exhibitions/birds-of-mughal-india/ |access-date=6 June 2024 |website=www.shangrilahawaii.org}}</ref><ref>{{Cite web |title=On Birds and Beauty - The Metropolitan Museum of Art |url=https://www.metmuseum.org/articles/concourse-of-the-birds |access-date=6 June 2024 |website=www.metmuseum.org |date=10 August 2016 |language=en}}</ref>

Among the most famous of these bird artists was [[John James Audubon]], whose paintings of [[List of North American birds|North American birds]] were a great commercial success in Europe and who later lent his name to the [[National Audubon Society]].<ref>{{cite journal | last1=Boime | first1=Albert | title=John James Audubon: a birdwatcher's fanciful flights | journal=Art History | volume=22 | pages=728–755 | year=1999 |doi=10.1111/1467-8365.00184 | issue=5}}</ref> Birds are also important figures in poetry; for example, [[Homer]] incorporated [[nightingale]]s into his ''[[Odyssey]]'', and [[Catullus]] used a [[Old World sparrow|sparrow]] as an erotic symbol in his [[Catullus 2]].<ref>{{Cite journal | last1=Chandler | first1=A. | year=1934 | title=The Nightingale in Greek and Latin Poetry | url=https://archive.org/details/sim_classical-journal_1934-11_30_2/page/78 | journal=The Classical Journal | volume=30 | issue=2| pages=78–84 | jstor=3289944 }}</ref> The relationship between an [[albatross]] and a sailor is the central theme of [[Samuel Taylor Coleridge]]'s ''[[The Rime of the Ancient Mariner]]'', which led to the use of the [[Albatross (metaphor)|term as a metaphor for a 'burden']].<ref>{{cite journal | last1=Lasky | first1=E. D. | title=A Modern Day Albatross: The Valdez and Some of Life's Other Spills | journal=The English Journal | volume=81 | issue=3 | pages=44–46 | date=March 1992 |doi=10.2307/820195 | jstor=820195}}</ref> Other [[English language|English]] metaphors derive from birds; [[vulture fund]]s and vulture investors, for instance, take their name from the scavenging vulture.<ref>{{Cite journal | last1=Carson | first1=A. | year=1998 | title=Vulture Investors, Predators of the 90s: An Ethical Examination | journal=Journal of Business Ethics | volume=17 | issue=5| pages=543–555 | doi=10.1023/A:1017974505642 }}</ref> Aircraft, particularly military aircraft, are frequently named after birds. The predatory nature of raptors make them popular choices for fighter aircraft such as the [[F-16 Fighting Falcon]] and the [[Harrier Jump Jet]], while the names of seabirds may be chosen for aircraft primarily used by naval forces such as the [[HU-16 Albatross]] and the [[V-22 Osprey]].

[[File:Flag of Dominica.svg|thumb|right|The [[Flag of Dominica|flag]] of [[Dominica]] prominently features the [[Sisserou Parrot]], its national bird.]]

Perceptions of bird species vary across cultures. [[Owl]]s are associated with bad luck, [[witchcraft]], and death in parts of Africa,<ref>{{cite book |last1=Enriquez |first1=P. L. |last2=Mikkola |first2=H. |year=1997 |chapter=Comparative study of general public owl knowledge in Costa Rica, Central America and Malawi, Africa |pages=160–166 |editor-first1=J. R. |editor-last1=Duncan |editor-first2=D. H. |editor-last2=Johnson |editor-first3=T. H. |editor-last3=Nicholls |title=Biology and conservation of owls of the Northern Hemisphere. General Technical Report NC-190 |publisher=USDA Forest Service |location=St. Paul, Minnesota}}</ref> but are regarded as wise across much of Europe.<ref>{{cite web |last1=Lewis |first1=DP |year=2005 |url=http://www.owlpages.com/articles.php?section=Owl+Mythology&title=Myth+and+Culture |website=Owlpages.com |title=Owls in Mythology and Culture |access-date=15 September 2007}}</ref> [[Hoopoe]]s were considered sacred in [[Ancient Egypt]] and symbols of virtue in [[Persia]], but were thought of as thieves across much of Europe and harbingers of war in [[Scandinavia]].<ref>{{Cite journal | last1=Dupree | first1=N. | year=1974 | title=An Interpretation of the Role of the Hoopoe in Afghan Folklore and Magic | journal=Folklore | volume=85 | issue=3| pages=173–193 | jstor=1260073 | doi=10.1080/0015587X.1974.9716553 }}</ref> In [[heraldry]], birds, especially [[Eagle (heraldry)|eagles]], often appear in [[coats of arms]]<ref>{{cite book |author=Fox-Davies, A. C. |title=A Complete Guide to Heraldry |publisher=Bloomsbury |date=1985}}</ref> In [[vexillology]], birds are a popular choice on [[List of national flags by design#Birds|flags]]. Birds feature in the flag designs of 17 countries and numerous subnational entities and territories.<ref>{{cite web |title=Flag description - the World Factbook |url=https://www.cia.gov/the-world-factbook/field/flag-description/ |archive-url=https://web.archive.org/web/20210301052948/https://www.cia.gov/the-world-factbook/field/flag-description/ |archive-date=2021-03-01}}</ref> Birds are used by nations to symbolise a country's identity and heritage, with 91 countries officially recognising a [[List of national birds|national bird]]. Birds of prey are highly represented, though some nations have chosen other species of birds with parrots being popular among smaller, tropical nations.<ref>{{cite web | url=https://www.vedantu.com/blog/national-birds-of-all-countries | title=List of National Birds of All Countries }}</ref>

===In music===
{{Main|Birds in music}}

[[Birds in music|In music]], birdsong has influenced composers and musicians in several ways: they can be inspired by birdsong; they can intentionally imitate bird song in a composition, as [[Antonio Vivaldi|Vivaldi]], [[Olivier Messiaen|Messiaen]], and [[Beethoven]] did, along with many later composers; they can incorporate recordings of birds into their works, as [[Ottorino Respighi]] first did; or like [[Beatrice Harrison]] and [[David Rothenberg]], they can duet with birds.<ref>{{cite journal |author=Head, Matthew |title=Birdsong and the Origins of Music |journal=Journal of the Royal Musical Association |volume=122 |issue=1 |year=1997 |pages=1–23 |doi=10.1093/jrma/122.1.1}}</ref><ref>{{cite book|author=Clark, Suzannah |year=2001 |title=Music Theory and Natural Order from the Renaissance to the Early Twentieth Century |publisher=Cambridge University Press |isbn=0-521-77191-9}}</ref><ref name=ledger>{{cite news |last=Reich |first=Ronni |title=NJIT professor finds nothing cuckoo in serenading our feathered friends |url=http://www.nj.com/entertainment/music/index.ssf/2010/10/njit_professor_finds_nothing_c.html |access-date=19 June 2011 |newspaper=Star Ledger |date=15 October 2010}}</ref><ref>{{Cite journal |last=Taylor |first=Hollis |date=21 March 2011 |title=Composers' Appropriation of Pied Butcherbird Song: Henry Tate's "undersong of Australia" Comes of Age |url=http://www.jmro.org.au/index.php/mca2/article/view/43 |journal=Journal of Music Research Online |volume=2 |archive-date=4 August 2017 |access-date=26 June 2017 |archive-url=https://web.archive.org/web/20170804113556/http://www.jmro.org.au/index.php/mca2/article/view/43 }}</ref>

A 2023 archaeological excavation of a 10,000-year-old site in Israel yielded hollow wing bones of coots and ducks with perforations made on the side that are thought to have allowed them to be used as flutes or whistles possibly used by [[Natufian culture|Natufian]] people to lure birds of prey.<ref>{{Cite journal |last1=Davin |first1=Laurent |last2=Tejero |first2=José-Miguel |last3=Simmons |first3=Tal |last4=Shaham |first4=Dana |last5=Borvon |first5=Aurélia |last6=Tourny |first6=Olivier |last7=Bridault |first7=Anne |last8=Rabinovich |first8=Rivka |last9=Sindel |first9=Marion |last10=Khalaily |first10=Hamudi |last11=Valla |first11=François |date=2023 |title=Bone aerophones from Eynan-Mallaha (Israel) indicate imitation of raptor calls by the last hunter-gatherers in the Levant |journal=Scientific Reports |volume=13 |issue=1 |page=8709 |doi=10.1038/s41598-023-35700-9 |pmc=10256695 |pmid=37296190|bibcode=2023NatSR..13.8709D }}</ref>

== Threats and conservation ==
{{Main|Bird conservation}}

{{See also|Late Quaternary prehistoric birds|List of extinct birds|Raptor conservation}}
[[File:California-Condor3-Szmurlo edit.jpg|thumb|right|upright|alt=Large black bird with featherless head and hooked bill|The [[California condor]] once numbered only 22 birds, but conservation measures have raised that to over 500 today.]]

Human activities have caused population decreases or [[extinction]] in many bird species. Over a hundred bird species have gone extinct in historical times,<ref>{{cite book |last1=Fuller |first1=Errol |year=2000 |title=Extinct Birds |edition=2nd |publisher=[[Oxford University Press]] |location=Oxford & New York |isbn=0-19-850837-9}}</ref> although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of [[Melanesia]]n, [[Polynesia]]n, and [[Micronesia]]n islands.<ref>{{cite book |last1=Steadman |first1=D. |year=2006 |title=Extinction and Biogeography in Tropical Pacific Birds |publisher=University of Chicago Press |isbn=978-0-226-77142-7}}</ref> Many bird populations are declining worldwide, with 1,227 species listed as [[threatened species|threatened]] by [[BirdLife International]] and the [[IUCN]] in 2009.<ref>{{cite news |title=BirdLife International announces more Critically Endangered birds than ever before |work=BirdLife |publisher=[[BirdLife International]] |date=14 May 2009 |url=http://www.birdlife.org/news/pr/2009/05/red_list.html |access-date=15 May 2009 |archive-date=17 June 2013 |archive-url=https://web.archive.org/web/20130617183344/http://www.birdlife.org/news/pr/2009/05/red_list.html }}</ref><ref>{{cite news|url=https://news.bbc.co.uk/1/hi/sci/tech/8045971.stm |first=Mark |last=Kinver |title=Birds at risk reach record high |work=BBC News Online |date=13 May 2009 |access-date=15 May 2009}}</ref> There have been long-term declines in North American bird populations, with an estimated loss of 2.9 billion breeding adults, about 30% of the total, since 1970.<ref>{{cite web |title=Nearly 3 Billion Birds Gone |url=https://www.birds.cornell.edu/home/bring-birds-back/ |website=CornellLab |access-date=5 May 2024}}</ref><ref>{{cite web |title=Vanishing: More Than 1 in 4 Birds Has Disappeared in the Last 50 Years |first=Gustave |last=Axelson |date=19 September 2019 |url=https://www.allaboutbirds.org/news/vanishing-1-in-4-birds-gone/ |website=All About Birds |access-date=5 May 2024}}</ref>

The most commonly cited human threat to birds is [[Habitat destruction|habitat loss]].<ref>{{cite book |editor-last1=Norris |editor-first1=K |editor-last2=Pain |editor-first2=D |year=2002 |title=Conserving Bird Biodiversity: General Principles and their Application |publisher=Cambridge University Press |isbn=978-0-521-78949-3}}</ref> Other threats include overhunting, accidental mortality due to collisions with [[bird-skyscraper collisions|buildings]] or [[bird strike|vehicles]], [[long-line fishing]] [[bycatch]],<ref>{{Cite journal | doi=10.1016/0006-3207(91)90031-4 | last1=Brothers | first1=N. P. | year=1991 | title=Albatross mortality and associated bait loss in the Japanese longline fishery in the southern ocean | journal=Biological Conservation | volume=55 | issue=3| pages=255–268 | bibcode=1991BCons..55..255B }}</ref> pollution (including [[oil spill]]s and pesticide use),<ref>{{cite journal | last1=Wurster | first1=D. | last2=Wurster | first2=C. |author-link2=Charles Frederick Wurster| last3=Strickland | first3=W. | date=July 1965 | title=Bird Mortality Following DDT Spray for Dutch Elm Disease | journal=Ecology | volume=46 | issue=4| pages=488–499 | doi =10.2307/1934880 | jstor=1934880 | bibcode=1965Ecol...46..488W }}; {{cite journal | doi=10.1126/science.148.3666.90 | title=Bird Mortality after Spraying for Dutch Elm Disease with DDT | year=1965 | last1=Wurster | first1=C.F. | last2=Wurster | first2=D.H. | last3=Strickland | first3=W.N. | journal=Science | volume=148 | issue=3666 | pages=90–91 | pmid=14258730 | bibcode=1965Sci...148...90W }}</ref> competition and predation from nonnative [[invasive species]],<ref>{{cite journal | last1=Blackburn | first1=T. | last2=Cassey | first2=P. | last3=Duncan | first3=R. | last4=Evans | first4=K. | last5=Gaston | first5=K. | date=24 September 2004 | title=Avian Extinction and Mammalian Introductions on Oceanic Islands | journal=[[Science (journal)|Science]] | volume=305 | issue=5692| pages=1955–1958 | doi=10.1126/science.1101617 | pmid=15448269| bibcode=2004Sci...305.1955B }}</ref> and [[Climate change and birds|climate change]].

Governments and [[conservation biology|conservation]] groups work to protect birds, either by passing laws that [[In-situ conservation|preserve]] and [[ecological restoration|restore]] bird habitat or by establishing [[Ex-situ conservation|captive populations]] for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the [[California condor]] and [[Norfolk parakeet]].<ref>{{cite journal | doi=10.1017/S0030605306000950 | last1=Butchart | first1=S. | last2=Stattersfield | first2=A. | last3=Collar | first3=N | year=2006 | title=How many bird extinctions have we prevented? | journal=Oryx | volume=40 | issue=3 | pages=266–79 | doi-access=free }}</ref>

Human activities have allowed the expansion of a few temperate area species, such as the [[barn swallow]] and [[European starling]]. In the tropics and sub-tropics, relatively more species are expanding due to human activities, particularly due to the spread of crops such as rice whose expansion in south Asia has benefitted at least 64 bird species, though may have harmed many more species.<ref>{{Cite journal |last1=Sundar |first1=K. S. Gopi |last2=Subramanya |first2=S. |date=2010 |title=Bird use of rice fields in the Indian subcontinent |journal=Waterbirds |volume=33 |issue=Special Issue 1 |pages=44–70 |doi=10.1675/063.033.s104 |bibcode=2010WaBir..33...44S }}</ref>

==See also==
* [[Biodiversity loss]]
* [[Breeding behaviors of birds]]
* [[Climate change and birds]]
* [[Glossary of bird terms]]
* [[List of individual birds]]
* [[Ornithology]]
* [[List of bird genera]]
{{Clear}}

== References ==
{{Reflist}}

==Further reading==
* ''All the Birds of the World'', Lynx Edicions, 2020.
* Del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi (eds.). ''Handbook of the Birds of the World'' (17-volume encyclopaedia), Lynx Edicions, Barcelona, 1992–2010. (''Vol. 1: Ostrich to Ducks'': {{ISBN|978-84-87334-10-8}}, etc.).
* Lederer, Roger; Carol Burr (2014). ''Latein für Vogelbeobachter: über 3000 ornithologische Begriffe erklärt und erforscht'', aus dem Englischen übersetzt von Susanne Kuhlmannn-Krieg, Verlag DuMont, Köln, {{ISBN|978-3-8321-9491-8}}.
* ''National Geographic Field Guide to Birds of North America'', National Geographic, 7th edition, 2017. {{ISBN|9781426218354}}
* ''National Audubon Society Field Guide to North American Birds: Eastern Region'', National Audubon Society, Knopf.
* ''National Audubon Society Field Guide to North American Birds: Western Region'', National Audubon Society, Knopf.
* Svensson, Lars (2010). ''Birds of Europe'', Princeton University Press, second edition. {{ISBN|9780691143927}}
* Svensson, Lars (2010). ''Collins Bird Guide: The Most Complete Guide to the Birds of Britain and Europe'', Collins, 2nd edition. {{ISBN|978-0007268146}}

==External links==
{{Sister project links|voy=no|wikt=bird|q=Birds|commons=Aves|species=Aves}}
{{Wikibooks|Dichotomous Key|Aves}}
{{Spoken Wikipedia|Bird_(Intro).ogg|date=5 January 2008}}
{{Library resources box
 |onlinebooks=yes
 |by=no
 |lcheading= Birds
 |label=Bird
 }}
* [http://avesbiology.com Avian Biology]
* [http://www.birdlife.org/ Birdlife International] – Dedicated to bird conservation worldwide; has a database with about 250,000 records on endangered bird species.
* [http://people.eku.edu/ritchisong/birdbiogeography1.htm Bird biogeography]
* [https://web.archive.org/web/20100810134957/http://www.audubon.org/bird/index.html Birds and Science] from the [[National Audubon Society]]
* [https://birdsoftheworld.org/bow/home Birds of the World, Cornell Lab]
* [https://www.birds.cornell.edu/ Cornell Lab of Ornithology]
* {{EOL}}
* [http://www.stanford.edu/group/stanfordbirds/text/essays/completed_essays.html Essays on bird biology]
* [http://www.mrnussbaum.com/birdsindex.htm North American Birds for Kids] {{Webarchive|url=https://web.archive.org/web/20100809090128/http://www.mrnussbaum.com/birdsindex.htm |date=9 August 2010 }}
* [http://www.ornithology.com/ Ornithology]
* [http://sora.unm.edu/ Sora] – Searchable online research archive; Archives of the following ornithological journals ''[[The Auk]]'', ''[[Condor (journal)|Condor]]'', ''Journal of Field Ornithology''', ''North American Bird Bander'', ''Studies in Avian Biology'', ''Pacific Coast Avifauna'', and the ''[[the Wilson Bulletin|Wilson Bulletin]]''.
* [http://ibc.lynxeds.com/ The Internet Bird Collection] – A free library of videos of the world's birds
* [http://www.birdpop.org/ The Institute for Bird Populations, California]
* [https://web.archive.org/web/20100323064035/http://media.library.uiuc.edu/cgi/b/bib/bix-idx?c=bix%3Bcc%3Dbix%3Bsid%3D0c4f6243857204b94fcdebc6dce5d8b2%3Btype%3Dsimple%3Bpage%3Dbrowse%3Binst%3Dbix_10%3Bsort%3Dregion List of field guides to birds], from the International Field Guides database
* [http://www.rspb.org.uk/wildlife/birdidentifier/ RSPB bird identifier] {{Webarchive|url=https://web.archive.org/web/20131105112202/http://www.rspb.org.uk/wildlife/birdidentifier/ |date=5 November 2013 }} – Interactive identification of all UK birds
* [http://www.ucmp.berkeley.edu/diapsids/avians.html Are Birds Really Dinosaurs?] — University of California Museum of Paleontology.

{{Birds}}
{{Chordata}}
{{Paraves|B.|state=autocollapse}}
{{Dinosaurs by Continent}}
{{Birds in culture}}
{{Taxonbar|from=Q5113}}
{{Authority control}}

[[Category:Birds| ]]
[[Category:Animal classes]]
[[Category:Ornithurae]]
[[Category:Extant Late Cretaceous first appearances]]
[[Category:Extant Maastrichtian first appearances]]
[[Category:Feathered dinosaurs]]
[[Category:Animal taxa named by Carl Linnaeus]]