Bird: Difference between revisions

 

{{Automatic taxtbox

| name = Birds

| fossil_range = <br />{{fossilrange|72|0|[[Late Cretaceous]] – [[Holocene|present]],<ref name="kuhl2020">H Kuhl, C Frankl-Vilches, A Bakker, G Mayr, G Nikolaus, S T Boerno, S Klages, B Timmermann, M Gahr (2020) [https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msaa191/5891114 An unbiased molecular approach using 3'UTRs resolves the avian family-level tree of life]. ''Molecular Biology and Evolution''. https://doi.org/10.1093/molbev/msaa191</ref> 72–0 [[Year#SI prefix multipliers|Ma]]| | earliest=160.8}}

| image = <center><imagemap>

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></center>

| 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 = Extant [[Order (biology)|orders]] and temporal ranges of [[Crown group#Pan-group|total groups]]

*** [[Struthioniformes]] (Ostriches) – 20–0 Mya, Early [[Miocene]]–[[Holocene|present]]<ref name="Mayr2017">{{cite book |last1=Mayr |first1=Gerald |title=Avian Evolution: The Fossil Record of Birds and its Paleobiological Significance |date=2017 |publisher=Wiley-Blackwell}}</ref>

*** [[Rheiformes]] (Rheas) – 53?–0 Mya, Early [[Eocene]]?–present<ref name="Mayr2017" /><ref name="Woodburne2014">{{cite journal |last1=Woodburne |first1=Michael O. |last2=Goin |first2=Francisco J. |last3=Raigemborn |first3=Maria Sol |last4=Heizler |first4=Matt |last5=Gelfo |first5=Javier N. |last6=Oliveira |first6=Edison V. |title=Revised timing of the South American early Paleogene land mammal ages |journal=Journal of South American Earth Sciences |date=October 2014 |volume=54 |pages=109–119 |doi=10.1016/j.jsames.2014.05.003|bibcode=2014JSAES..54..109W }}</ref>

*** [[Tinamiformes]] (Tinamous) – 17.5–0 Mya, Early [[Miocene]]–present<ref>{{cite journal |last1=Bertelli |first1=Sara |last2=Chiappe |first2=Luis M. |title=Earliest tinamous (Aves: Palaeognathae) from the Miocene of Argentina and their phylogenetic position |journal=Contributions in Science |date=2005 |volume=502 |pages=1–20}}</ref>

*** [[Casuariiformes]] (Casowaries and Emus) – 24–0 Mya, Late [[Oligocene]]–present<ref name="Mayr2017" />

*** [[Apterygiformes]] (Kiwis) – 19–0 Mya, Early [[Miocene]]–present<ref name="Mayr2017" /><ref>{{cite journal |last1=Worthy |first1=Trevor H. |last2=Worthy |first2=Jennifer P. |last3=Tennyson |first3=Alan J. D. |last4=Salisbury |first4=Steven W. |last5=Hand |first5=Suzanne J. |last6=Scofield |first6=R. Paul |editor1-last=Göhlich |editor1-first=Ursula B. |editor2-last=Kroh |editor2-first=Andreas |title=Miocene fossils show that kiwi (Apteryx, Apterygidae) are probably not phyletic dwarves |journal=Paleornithological Research 2013: Proceedings of the 8th International Meeting of the Society of Avian Paleontology and Evolution |date=2013 |pages=63–80 |publisher=Naturhistorisches Museum}}</ref>

* Infraclass [[Neognathae]]

** Superorder [[Galloanserae]]

*** [[Galliformes]] (Gamebirds) – 55–0 Mya, Early [[Eocene]]–present<ref name="Mayr2009" />

*** [[Anseriformes]] (Waterfowl) – 71?–0 Mya, Late [[Cretaceous]] ([[Maastrichtian]])?–present<ref>{{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 |pages=150635 |doi=10.1098/rsos.150635|pmid=26998335 |pmc=4785986 |bibcode=2016RSOS....350635D |doi-access=free }}</ref>

** Superorder [[Neoaves]]

*** [[Phoenicopteriformes]] (Flamingos) – 38–0 Mya, Late [[Eocene]]–present<ref name="Mayr2009">{{cite book |last1=Mayr |first1=Gerald |title=Paleogene Fossil Birds |date=2009 |publisher=Springer}}</ref>

*** [[Podicipediformes]] (Grebes) – 28–0 Mya, Late [[Oligocene]]–present<ref name="Mayr2009" />

*** [[Columbiformes]] (Pigeons and doves) – 28–0 Mya, Late [[Oligocene]]–present<ref name="Mayr2017" /><ref>{{cite journal |last1=Worthy |first1=Trevor H. |title=A phabine pigeon (Aves:Columbidae) from Oligo-Miocene Australia |journal=Emu - Austral Ornithology |date=March 2012 |volume=112 |issue=1 |pages=23–31 |doi=10.1071/MU11061|s2cid=83994441 }}</ref>

*** [[Mesitornithiformes]] (Mesites) – No fossil record

*** [[Pterocliformes]] (Sandgrouse) – 41?–0 Mya, Middle [[Eocene]]?–present<ref name="Mayr2017" />

*** [[Apodiformes]] (Swifts, treeswifts and hummingbirds) – 55–0 Mya, Early [[Eocene]]–present<ref name="Mayr2009" />

*** [[Caprimulgiformes]] (Nightjars, nighthawks, potoos, oilbirds, frogmouths and owlet-nightjars) – 55–0 Mya, Early [[Eocene]]–present<ref name="Mayr2009" />

*** [[Cuculiformes]] (Cuckoos) – 38?–0 Mya, Late [[Eocene]]?–present<ref name="Mayr2009" />

*** [[Otidiformes]] (Bustards) – 11–0 Mya, Late [[Miocene]]–present<ref name="Mayr2017" /><ref>{{cite journal |last1=Boev |first1=Zlatozar |last2=Lazaridis |first2=Georgios |last3=Tsoukala |first3=Evangelia |title=''Otis hellenica'' sp. nov., a new Turolian bustard (Aves: Otididae) from Kryopigi (Chalkidiki, Greece) |journal=Geologica Balcanica |date=2013 |volume=42 |pages=59–65 |url=https://www.geologica-balcanica.eu/journal/42/1-3/pp-59-65}}</ref>

*** [[Musophagiformes]] (Turacos) – 52–0 Mya, Early [[Eocene]]–present<ref>{{cite journal |last1=Field |first1=Daniel J. |last2=Hsiang |first2=Allison Y. |title=A North American stem turaco, and the complex biogeographic history of modern birds |journal=BMC Evolutionary Biology |date=December 2018 |volume=18 |issue=1 |pages=102 |doi=10.1186/s12862-018-1212-3|pmid=29936914 |pmc=6016133 |doi-access=free }}</ref>

*** [[Opisthocomiformes]] (Hoatzin) – 34–0 Mya, Late [[Eocene]]–present<ref>{{cite journal |last1=Mayr |first1=Gerald |last2=De Pietri |first2=Vanesa L. |title=Earliest and first Northern Hemispheric hoatzin fossils substantiate Old World origin of a "Neotropic endemic" |journal=Naturwissenschaften |date=February 2014 |volume=101 |issue=2 |pages=143–148 |doi=10.1007/s00114-014-1144-8|pmid=24441712 |bibcode=2014NW....101..143M |s2cid=14060583 }}</ref>

*** [[Gruiformes]] (Cranes, crakes, rails, wood-rails, flufftails, gallinules, limpkins, trumpeters, finfoots and sungrebes) – 59–0 Mya, Late [[Paleocene]]–present<ref name="Mayr2017" />

*** [[Charadriiformes]] (Plovers, crab plovers, lapwings, gulls, puffins, auks, sandipipers, buttonquails, stilts, avocets, ibisbills, woodcocks, skuas, etc) – 55–0 Mya, Early [[Eocene]]–present<ref>{{cite journal |last1=Hood |first1=Sarah C. |last2=Torres |first2=Chris R. |last3=Norell |first3=Mark A. |last4=Clarke |first4=Julia A. |title=New Fossil Birds from the Earliest Eocene of Mongolia |journal=American Museum Novitates |date=9 August 2019 |volume=2019 |issue=3934 |pages=1 |doi=10.1206/3934.1|s2cid=199571350 |url=https://www.biodiversitylibrary.org/item/271741 }}</ref>

*** [[Gaviiformes]] (Loons) – 48–0 Mya, Middle [[Eocene]]–present<ref name="Mayr2017" /><ref>{{cite journal |last1=Mayr |first1=Gerald |last2=Zvonok |first2=Evgenij |title=Middle Eocene Pelagornithidae and Gaviiformes (Aves) from the Ukrainian Paratethys: MIDDLE EOCENE BIRDS FROM UKRAINE |journal=Palaeontology |date=November 2011 |volume=54 |issue=6 |pages=1347–1359 |doi=10.1111/j.1475-4983.2011.01109.x|doi-access=free }}</ref>

*** [[Procellariiformes]] (Petrels, storm petrels, and albatrosses) – 66?–0 Mya, Early [[Paleocene]]?–present<ref name="Mayr2017" />

*** [[Sphenisciformes]] (Penguins) – 62–0 Mya, Early [[Paleocene]]–present<ref name="Ksepka&Clarke2015">{{cite journal |last1=Ksepka |first1=Dt |last2=Clarke |first2=Ja |title=Phylogenetically vetted and stratigraphically constrained fossil calibrations within Aves |journal=Palaeontologia Electronica |date=2015 |doi=10.26879/373|doi-access=free }}</ref>

*** [[Ciconiiformes]] (Storks) – 30–0 Mya, Early [[Oligocene]]–present<ref name="Mayr2017" /><ref>{{cite journal |last1=Seiffert |first1=E. R. |title=Revised age estimates for the later Paleogene mammal faunas of Egypt and Oman |journal=Proceedings of the National Academy of Sciences |date=28 March 2006 |volume=103 |issue=13 |pages=5000–5005 |doi=10.1073/pnas.0600689103|pmid=16549773 |pmc=1458784 |bibcode=2006PNAS..103.5000S |doi-access=free }}</ref>

*** [[Pelecaniformes]] (Pelicans, ibises, shoebills, egrets, herons, etc) – 55–0 Mya, Early [[Eocene]]–present<ref name="Smith&Ksepka2015">{{cite journal |last1=Smith |first1=Nd |last2=Ksepka |first2=Dt |title=Five well-supported fossil calibrations within the "Waterbird" assemblage (Tetrapoda, Aves) |journal=Palaeontologia Electronica |date=2015 |doi=10.26879/483|doi-access=free }}</ref>

*** [[Phaethontiformes]] (Tropicbirds) – 62–0 Mya, Early [[Paleocene]]–present<ref>{{cite journal |last1=Mayr |first1=Gerald |last2=Scofield |first2=R. Paul |title=New avian remains from the Paleocene of New Zealand: the first early Cenozoic Phaethontiformes (tropicbirds) from the Southern Hemisphere |journal=Journal of Vertebrate Paleontology |date=2 January 2016 |volume=36 |issue=1 |pages=e1031343 |doi=10.1080/02724634.2015.1031343|s2cid=130026060 }}</ref>

*** [[Accipitriformes]] (Eagles, Old World vultures, secretary-birds, hawks, harriers, etc) – 52–0 Mya, Early [[Eocene]]–present<ref>{{cite journal |last1=Mayr |first1=Gerald |last2=Smith |first2=Thierry |title=A diverse bird assemblage from the Ypresian of Belgium furthers knowledge of early Eocene avifaunas of the North Sea Basin |journal=Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen |date=22 March 2019 |volume=291 |issue=3 |pages=253–281 |doi=10.1127/njgpa/2019/0801}}</ref>

*** [[Strigiformes]] (Owls) – 60–0 Mya, Middle [[Paleocene]]–present<ref name="Mayr2009" />

*** [[Leptosomiformes]] (Cuckoorollers) – 56.5?–0 Mya, Late [[Paleocene]]?–present<ref>{{cite journal |last1=Mayr |first1=Gerald |last2=Smith |first2=Thierry |title=New Paleocene bird fossils from the North Sea Basin in Belgium and France |journal=Geologica Belgica |date=2019 |volume=22 |issue=1–2 |pages=35–46 |doi=10.20341/gb.2019.003|doi-access=free }}</ref>

*** [[Bucerotiformes]] (Hornbills, hoopoes and wood-hoopoes) – 55–0 Mya, Early [[Eocene]]–present<ref name="Mayr2009" />

*** [[Coraciiformes]] (Rollers, bee eaters, todies, kingfishers, etc) – 55–0 Mya, Early [[Eocene]]–present<ref name="Smith&Ksepka2015" /><ref>{{cite journal |last1=Bourdon |first1=Estelle |last2=Kristoffersen |first2=Anette V. |last3=Bonde |first3=Niels |title=A roller-like bird (Coracii) from the Early Eocene of Denmark |journal=Scientific Reports |date=December 2016 |volume=6 |issue=1 |pages=34050 |doi=10.1038/srep34050|pmid=27670387 |pmc=5037458 |bibcode=2016NatSR...634050B |doi-access=free }}</ref>

*** [[Piciformes]] (Woodpeckers, toucans, puffbirds, etc) – 47?–0 Mya, Middle [[Eocene]]?–present<ref name="Mayr2017" />

*** [[Cariamiformes]] (Seriemas) – 53–0 Mya, Early [[Eocene]]–present<ref name="Mayr2017" /><ref name="Woodburne2014" /><ref name="Oliveros2019">{{cite journal |last1=Oliveros |first1=Carl H. |last2=Field |first2=Daniel J. |last3=Ksepka |first3=Daniel T. |last4=Barker |first4=F. Keith |last5=Aleixo |first5=Alexandre |last6=Andersen |first6=Michael J. |last7=Alström |first7=Per |last8=Benz |first8=Brett W. |last9=Braun |first9=Edward L. |last10=Braun |first10=Michael J. |last11=Bravo |first11=Gustavo A. |last12=Brumfield |first12=Robb T. |last13=Chesser |first13=R. Terry |last14=Claramunt |first14=Santiago |last15=Cracraft |first15=Joel |last16=Cuervo |first16=Andrés M. |last17=Derryberry |first17=Elizabeth P. |last18=Glenn |first18=Travis C. |last19=Harvey |first19=Michael G. |last20=Hosner |first20=Peter A. |last21=Joseph |first21=Leo |last22=Kimball |first22=Rebecca T. |last23=Mack |first23=Andrew L. |last24=Miskelly |first24=Colin M. |last25=Peterson |first25=A. Townsend |last26=Robbins |first26=Mark B. |last27=Sheldon |first27=Frederick H. |last28=Silveira |first28=Luís Fábio |last29=Smith |first29=Brian Tilston |last30=White |first30=Noor D. |last31=Moyle |first31=Robert G. |last32=Faircloth |first32=Brant C. |title=Earth history and the passerine superradiation |journal=Proceedings of the National Academy of Sciences |date=16 April 2019 |volume=116 |issue=16 |pages=7916–7925 |doi=10.1073/pnas.1813206116|pmid=30936315 |pmc=6475423 |doi-access=free }}</ref>

*** [[Falconiformes]] (Falcons and caracaras) – 53?–0 Mya, Early [[Eocene]]?–present<ref name="Mayr2017" /><ref>{{cite journal |last1=Cenizo |first1=Marcos |last2=Noriega |first2=Jorge I. |last3=Reguero |first3=Marcelo A. |title=A stem falconid bird from the Lower Eocene of Antarctica and the early southern radiation of the falcons |journal=Journal of Ornithology |date=July 2016 |volume=157 |issue=3 |pages=885–894 |doi=10.1007/s10336-015-1316-0|s2cid=15517037 |url=http://sedici.unlp.edu.ar/handle/10915/107977 }}</ref>

*** [[Psittaciformes]] (Parrots) – 55–0 Mya, Early [[Eocene]]–present<ref name="Mayr2009" />

*** [[Passeriformes]] (Passerines) – 55–0 Mya, Early [[Eocene]]–present<ref name="Mayr2009" /><ref name="Oliveros2019" />

'''Birds''' are a group of [[warm-blooded]] [[vertebrate]]s constituting the [[class (biology)|class]] '''Aves''' {{IPAc-en|ˈ|eɪ|v|iː|z}}, characterised by [[feather]]s, toothless 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 {{convert|5.5|cm|in|sigfig=2|abbr=on}} [[bee hummingbird]] to the {{convert|2.8|m|ftin|sigfig=2|abbr=on}} [[Common ostrich|ostrich]]. There are about ten thousand living species, more than half of which are [[passerine]], or "perching" birds. Birds have {{Birdgloss|wings}} whose development varies according to species; the only known groups without wings are the extinct [[moa]] and [[elephant bird]]s. Wings, which evolved from [[forelimb]]s, gave birds the ability to fly, although further evolution has led to the 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.

Birds are a group of [[feathered dinosaur|feathered]] [[theropod]] [[dinosaur]]s and constitute the [[Origin of birds|only living dinosaurs]]. Likewise, birds are considered [[reptile]]s in the modern [[Cladistics|cladistic]] sense of the term, and their closest living relatives are the [[crocodilia]]ns. Birds are descendants of the primitive [[Avialae|avialans]] (whose members include ''[[Archaeopteryx]]'') which first appeared about 160&nbsp;million years ago (mya) in China. According to DNA evidence, modern birds ('''Neornithes''') evolved in the [[Middle Cretaceous|Middle]] to [[Late Cretaceous]], and diversified dramatically around the time of the [[Cretaceous–Paleogene extinction event]] 66 mya, which killed off the [[pterosaur]]s and all non-avian dinosaurs.

Many [[social animal|social species]] pass on knowledge across generations, which is considered [[Animal culture#Examples of culturally transmitted behaviors in birds|a form of culture]]. Birds are social, communicating with visual signals, calls, and [[bird vocalization|songs]], and participating in such behaviours 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, but 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.

[[File:Archaeopteryx_lithographica_(Berlin_specimen).jpg|alt= Slab of stone with fossil bones and feather impressions|thumb|right|''[[Archaeopteryx lithographica]]'' 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 categorised as the [[Class (biology)|biological class]] Aves in [[Linnaean taxonomy]]. [[Phylogenetic taxonomy]] places Aves in the dinosaur [[clade]] [[Theropoda]].<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>

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=L.M. Chiappe |author3=Chiappe LM |editor=[[Philip J. Currie]] |editor2=Kevin Padian |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 of the [[Phylocode]] system. 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>{{Cite book|last=Gauthier |first=Jacques|editor=Kevin Padian |title=The Origin of Birds and the Evolution of Flight|series= Memoirs of the California Academy of Science '''8'''|year=1986|pages=1–55|chapter=Saurischian Monophyly and the origin of birds|isbn=0-940228-14-9 |publisher=Published by California Academy of Sciences |location=San Francisco, CA}}</ref> in part to avoid the uncertainties about the placement of ''Archaeopteryx'' in relation to animals traditionally thought of as theropod dinosaurs.<!-- See WP:RS<ref>http://www.phylonames.org/forum/viewtopic.php?t=7</ref> --><!--Mayr et al. 2005 "A well-preserved Archaeopteryx specimen with theropod features" + comment + Mayr's comment on the comment-->

Gauthier and de Queiroz<ref name="gauthier&dequeiroz2001"/> identified four different definitions for the same biological name "Aves", which is a problem. 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, which corresponds to meaning number 4 below. He assigned other names to the other groups.

{{Cladogram|caption=The birds' phylogenetic relationships to major living reptile groups.

|clades={{clade|style=font-size:75%

  |1= {{clade

    |1= {{clade

      |1= [[Crocodile]]s

      |2= '''Birds'''

    |2= [[Turtle]]s

  |2= [[Lizard]]s (including [[snake]]s)

}}

# 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=Pascal Godefroit |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|s2cid=4364892 |author1-link=Pascal Godefroit }}</ref>

{{Cladogram|caption=Cladogram showing the results of a phylogenetic study by Cau, 2018.<ref name="paleoitalia"/>|clades=

      |1=†''[[Coelurus]]''

      |2={{clade

        |1=†''[[Ornitholestes]]''

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">Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.) (2004). ''The Dinosauria'', Second Edition. University of California Press., 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 | s2cid = 83726237 }}</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|s2cid=55462557|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 [[wing]]s used in flapping [[Bird flight|flight]], and the birds that descended from them.<ref name="gauthier&dequeiroz2001">Gauthier, J., and de Queiroz, K. (2001). "Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves." pp. 7–41 in ''New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom'' (J.A. Gauthier and L.F. Gall, eds.). Peabody Museum of Natural History, Yale University, New Haven, CT</ref><ref name="gauthier1986">Gauthier, J. (1986). "Saurischian monophyly and the origin of birds." In: K. Padian, ed. ''The origin of birds and the evolution of flight.'' San Francisco: California, Acad. Sci. pp. 1–55. (Mem. Calif. Acad. Sci.8.)</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>{{Citation|last1=Lee|first1=Michael S. Y.|title=CHAPTER 3 - CROWN-CLADES, KEY CHARACTERS AND TAXONOMIC STABILITY: WHEN IS AN AMNIOTE NOT AN AMNIOTE?|date=1 January 1997|url=http://www.sciencedirect.com/science/article/pii/B9780126764604500044|work=Amniote Origins|pages=61–84|editor-last=Sumida|editor-first=Stuart S.|publisher=Academic Press|language=en|isbn=978-0-12-676460-4|access-date=14 May 2020|last2=Spencer|first2=Patrick S.|editor2-last=Martin|editor2-first=Karen L. M.}}</ref>

===Dinosaurs and the origin of birds===

{{Main|Origin of birds}}

[[File:Anchiornis feathers.jpg|thumb|upright=0.8|left|''[[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|s2cid=206525132 |url=http://doc.rero.ch/record/210394/files/PAL_E4402.pdf }}</ref>]]

{{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|pages=e1032|year=2015|last1=Cau|first1=Andrea|last2=Brougham|first2=Tom|last3=Naish|first3=Darren}}</ref>|clades=

    |2=†''[[Aurornis]]''

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. Prum|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|s2cid=206517571|doi-access=free}}</ref> and more specifically, they are 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 |location=Baltimore|publisher=Johns Hopkins University Press |isbn=0-8018-6763-0|pages=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. Recent discoveries in the [[Liaoning]] Province of northeast China, which demonstrate many small theropod [[feathered dinosaurs]], contribute 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 |url-status=dead }}</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|s2cid=37866029 }}</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|s2cid=205225790 }}</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. |date=7 September 2007 |title=A basal dromaeosaurid and size evolution preceding avian flight |url=http://www.sciencemag.org/cgi/reprint/317/5843/1378.pdf |journal=[[Science (journal)|Science]]|volume=317 |pages=1378–1381 |doi=10.1126/science.1144066 |pmid=17823350 |issue=5843 |last2=Pol |first2=D. |last3=Clarke |first3=J.A. |last4=Erickson |first4=G.M. |last5=Norell |first5=M.A. |bibcode=2007Sci...317.1378T |s2cid=2519726 |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|s2cid=1160118}}</ref> Unlike ''Archaeopteryx'' and the non-avialan feathered dinosaurs, who primarily ate meat, recent 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 | url-status=dead}}</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>

[[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>Ivanov, M., Hrdlickova, S. & Gregorova, R. (2001) The Complete Encyclopedia of Fossils. Rebo Publishers, Netherlands. p. 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|journal=Bollettino della Società Paleontologica Italiana}}</ref><ref name="sciencedirect">{{Cite journal|last1=Benton|first1=Michael J.|last2=Dhouailly|first2=Danielle|last3=Jiang|first3=Baoyu|last4=McNamara|first4=Maria|date=1 September 2019|title=The Early Origin of Feathers|url=http://www.sciencedirect.com/science/article/pii/S0169534719301405|journal=Trends in Ecology & Evolution|language=en|volume=34|issue=9|pages=856–869|doi=10.1016/j.tree.2019.04.018|pmid=31164250|issn=0169-5347}}</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 feathers.<ref name="sciencedirect"/>

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 manoeuvreing.<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| citeseerx = 10.1.1.1031.5732 | bibcode = 2013Sci...339.1309Z | s2cid = 206544531 }}</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 | url-status=dead | 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=

    |label1=[[Avialae]]

        |1=†''[[Anchiornis]]''

        |1=†''[[Archaeopteryx]]''

===Early diversity of bird ancestors===

[[File:Ichthyornis Clean.png|thumb|upright=0.8|left|''[[Ichthyornis]]'', which lived 93&nbsp;million years ago, was the first known prehistoric bird relative preserved with teeth.]]

| bibcode = 2015NatCo...6.6987W| pmc = 5426517

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"/>

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 euornithes lacked [[Perching bird|perching]] adaptations and seem to have included shorebird-like species, waders, and swimming and diving species.

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|url=http://digitallibrary.amnh.org/dspace/bitstream/2246/454/1/B286.pdf|hdl=2246/454}}</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 euornithes 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 euornithes retained teeth in other parts of the jaws).<ref name=louchart2011>{{cite journal | last1 = Louchart | first1 = A. | last2 = Viriot | first2 = L. | year = 2011 | title = From snout to beak: the loss of teeth in birds | url = http://198.81.200.84/trends/ecology-evolution/abstract/S0169-5347%2811%2900264-3?switch=standard | journal = Trends in Ecology & Evolution | volume = 26 | issue = 12 | pages = 663–673 | doi = 10.1016/j.tree.2011.09.004 | pmid = 21978465 | url-status = dead | archive-url = https://web.archive.org/web/20140728053547/http://198.81.200.84/trends/ecology-evolution/abstract/S0169-5347%2811%2900264-3?switch=standard | archive-date = 28 July 2014}}</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 [[omnivore]]s, 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 }}</ref>

===Diversification of modern birds===

{{See also|Sibley–Ahlquist taxonomy of birds|dinosaur classification}}

{{Cladogram|caption=Basal divergences of modern birds<br />based on [[Sibley-Ahlquist taxonomy]]

|clades={{clade | style=font-size:75%;line-height:80%

|label1='''Aves'''

|label1=[[Palaeognathae]]

|1={{clade

    |1=[[Struthioniformes]]

    |2=[[Tinamiformes]]

|label2= [[Neognathae]]

|2={{clade

    |1=Other birds ([[Neoaves]])

    |label1=

    |label2=[[Galloanserae]]

        |1=[[Anseriformes]]

        |2=[[Galliformes]]

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| s2cid = 206555952| url = http://dspace.flinders.edu.au/xmlui/bitstream/2328/35953/1/Mitchell_AncientDNA_AM2014.pdf| hdl-access = free}}</ref> These two subdivisions are often 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> although [[Bradley C. Livezey|Livezey]] and Zusi assigned them "cohort" rank.<ref name="Theropoda"/> Depending on the [[alpha taxonomy|taxonomic]] viewpoint, the number of known living bird species varies anywhere from 9,800<ref>{{Cite book|title=The Clements Checklist of Birds of the World |first=James F. |last=Clements |edition=6th |author-link=James Clements |location=Ithaca |publisher=[[Cornell University Press]] |year=2007 |isbn=978-0-8014-4501-9|title-link=The Clements Checklist of Birds of the World }}</ref> to 10,758.<ref>{{Cite journal|doi=10.14344/ioc.ml.9.2|title=Welcome |journal=IOC World Bird List 9.2}}</ref>

The discovery of ''[[Vegavis]]'' from the [[Maastrichtian]], the last stage of the Late Cretaceous proved that the diversification of modern birds started before the [[Cenozoic|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 |s2cid=4354309 }}</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) | url = http://digitallibrary.amnh.org/dspace/bitstream/handle/2246/454/B286.?sequence=1 | 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 }}</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=":0">{{Cite journal|last1=Field|first1=Daniel J.|last2=Benito|first2=Juan|last3=Chen|first3=Albert|last4=Jagt|first4=John W. M.|last5=Ksepka|first5=Daniel T.|date=18 March 2020|title=Late Cretaceous neornithine from Europe illuminates the origins of crown birds|url=https://www.repository.cam.ac.uk/handle/1810/303639|journal=Nature|language=en|volume=579|issue=7799|pages=397–401|doi=10.1038/s41586-020-2096-0|issn=1476-4687|pmid=32188952}}</ref>

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the [[Middle Cretaceous]]<ref name=divergence>{{Cite journal | last1 = Lee | first1 = Michael SY | last2 = Cau | first2 = Andrea | last3 = Naish | first3 = Darren | last4 = Dyke | first4 = Gareth J. | date = May 2014 | title = Morphological Clocks in Paleontology, and a Mid-Cretaceous Origin of Crown Aves | journal = Systematic Biology | publisher = Oxford Journals | doi = 10.1093/sysbio/syt110 | volume=63 |issue=1 | pages=442–449 | pmid=24449041| url = https://academic.oup.com/sysbio/article-pdf/63/3/442/9164850/syt110.pdf | doi-access = free }}</ref> to the latest [[Late Cretaceous]].<ref name="kuhl2020"/><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 | s2cid = 205246158 }}</ref> Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after 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=CL |last3=Britton |first3=T |last4=Elzanowski |first4=A |last5=Johansson |first5=US |last6=Källersjö |first6=M |last7=Ohlson |first7=JI |last8=Parsons |first8=TJ |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 |url-status=dead }}</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). Previous attempts to reconcile molecular and fossil evidence have proved controversial,<ref name="Ericson"/><ref>{{Cite journal|last1=Brown |first1=Joseph W. |date=June 2007 |title=Nuclear DNA does not reconcile 'rocks' and 'clocks' in Neoaves: a comment on Ericson et al. |journal=[[Biology Letters]] |volume=3 |issue=3 |pages=257–259 |doi=10.1098/rsbl.2006.0611 |pmid=17389215 |last2=Payne |first2=RB |last3=Mindell |first3=DP |pmc=2464679}}</ref> but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating [[molecular clocks]], showed that while modern birds originated early in the Late Cretaceous in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds expanded from West Gondwana to the Laurasia through two routes. One route was an Antarctic interchange in the Paleogene. This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. The other route was probably through North American, via land bridges during the Paleocene. This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics.<ref name=cracraft>{{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 |page=e1501005|bibcode=2015SciA....1E1005C }}</ref>

[[Cladogram]] of modern bird relationships based on Kuhl, H. ''et al.'' (2020)<ref name="kuhl2020"/>

  |1=[[Palaeognathae]] ([[Ostrich]]es and relatives) [[File:Struthio camelus - Etosha 2014 (1) white background.jpg|50 px]]

                        |2=[[Caprimulgiformes]] ([[Swift (bird)|swift]]s, [[hummingbird]]s, [[nightjar]]s and allies)[[File:Haaksnavelkolibrie.jpg|50 px]]

                          |2=[[Musophagiformes]] (turacos)[[File:Planches enluminées d'histoire naturelle (1765) (Tauraco persa).jpg|50 px]]

                              |1=[[Cuculiformes]] (cuckoos)[[File:British birds in their haunts (Cuculus canorus).jpg|50 px]]

                        |2=[[Eurypygiformes]] ([[sunbittern]] and [[kagu]])[[File:Cuvier-72-Caurale soleil.jpg|50 px]]

                                |1=[[Suliformes]] ([[Booby|boobies]], [[cormorant]]s, etc.) [[File:Cormorant in Strunjan, white background.png|70px]]

                                    |1=[[Trogoniformes]] (trogons and quetzals)[[File:Harpactes fasciatus 1838 white background.jpg|40 px]]

                                      |1=[[Bucerotiformes]] ([[hornbill]]s and relatives)[[File:A monograph of the Bucerotidæ, or family of the hornbills (Plate II) (white background).jpg|50 px]]

                                          |1=[[Coraciiformes]] ([[kingfisher]]s and relatives)[[File:Cuvier-46-Martin-pêcheur d'Europe.jpg|50 px]]

                              |2=[[Passeriformes]] (passerines)[[File:Cuvier-33-Moineau domestique.jpg|50 px]]

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 classification of birds,<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> although it is frequently debated and constantly revised. 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 between 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. More recently, new fossil and molecular evidence 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 = etal  | 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>

{{as of|2020}}, the [[genome]] has been sequenced for 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|issn=0028-0836|doi=10.1038/s41586-020-2873-9|pmid=33177665|pmc=7759463|bibcode=2020Natur.587..252F|doi-access=free}}</ref>

==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 recent studies 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|s2cid=46640620 |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 |url=http://jeb.biologists.org/cgi/content/full/205/9/1189|first2=Y|last2=Naito|first3=A|last3=Kato|first4=Y|last4=Niizuma|first5=Y|last5=Watanuki|first6=JB|last6=Charrassin|first7=CA|last7=Bost|first8=Y|last8=Handrich|first9=Y|last9=Le Maho|doi=10.1242/jeb.205.9.1189 }}</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=http://bna.birds.cornell.edu/bna/species/322 |doi=10.2173/bna.322 |access-date=13 December 2015}}</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 practices]] created suitable new habitat.

==Anatomy and physiology==

{{Main|Bird anatomy|Bird vision}}

{{See also|Egg tooth}}

[[File:Birdmorphology.svg|thumb|upright=1.35|alt= |right|External anatomy of a bird (example: [[yellow-wattled lapwing]]): 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser [[covert (feather)|coverts]], 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 [[Cloaca#Birds|Vent]], 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Foot, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 Wattle, 23 Eyestripe]]

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

===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/|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 the [[reptile]]s, birds are primarily 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 and (with 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 |s2cid=18540594 |doi-access=free }}</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|s2cid=4372695 }}</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#Birds|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|s2cid=85424175}}</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 |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 |url-status=dead}}</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|title=What Are Fecal Sacs? Bird Diapers, Basically|website=Audubon|access-date=17 January 2021}}</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/ |title=Phenomena: Not Exactly Rocket Science How Chickens Lost Their Penises (And Ducks Kept Theirs) |date=6 June 2013 |publisher=Phenomena.nationalgeographic.com |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 |url-status=dead |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> The length is thought to be related to [[sperm competition]].<ref>{{cite journal|journal=The Auk |volume=117 |issue=3 |pages=820–825 |year=2000 |title=The 20-cm Spiny Penis of the Argentine Lake Duck (Oxyura vittata) |author=McCracken, KG |url=http://sora.unm.edu/sites/default/files/journals/auk/v117n03/p00820-p00825.pdf |archive-url=https://web.archive.org/web/20160304210246/http://sora.unm.edu/sites/default/files/journals/auk/v117n03/p00820-p00825.pdf |archive-date=4 March 2016 |doi=10.1642/0004-8038(2000)117[0820:TCSPOT]2.0.CO;2 |url-status=dead}}</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|language=en|volume=59|issue=4|pages=334–338|doi=10.1262/jrd.2013-038|issn=0916-8818|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|date=2014|title=Gonadal Asymmetry and Sex Determination in Birds|url=https://www.karger.com/Article/FullText/358406|journal=Sexual Development|language=en|volume=8|issue=5|pages=227–242|doi=10.1159/000358406|pmid=24577119|s2cid=3035039|issn=1661-5433|doi-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 |s2cid=13704885 |doi-access=free }}</ref><ref>{{cite journal |last1=FARNER |first1=DONALD S. |last2=FOLLETT |first2=BRIAN K. |last3=KING |first3=JAMESR. |last4=MORTON |first4=MARTIN 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 |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" />

Birds are solely [[Gonochorism|gonochoric]].<ref>{{Cite book|last1=Kobayashi|first1=Kazuya|url=https://books.google.com/books?id=g4teDwAAQBAJ&q=mammal+gonochorism&pg=PA290|title=Reproductive and Developmental Strategies: The Continuity of Life|last2=Kitano|first2=Takeshi|last3=Iwao|first3=Yasuhiro|last4=Kondo|first4=Mariko|date=2018-06-01|publisher=Springer|isbn=978-4-431-56609-0|pages=290|language=en}}</ref> Meaning they have two sexes: either [[female]] or [[male]]. The sex of birds is determined by the [[ZW sex-determination system|Z and W sex chromosomes]], rather than by the [[XY sex-determination system|X and Y chromosomes]] present in [[mammal]]s. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).<ref name = "Gill"/>

In nearly all species of birds, an individual's sex is determined at fertilisation. However, one recent 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}}</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|pmc=1629050 | pmid=17148121|doi=10.1098/rsbl.2004.0247|volume=1|issue=1 |date=March 2005|pages=31–33 | last1 = Göth | first1 = A | last2 = Booth | first2 = DT | journal=Biology Letters}}</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=November 2006 |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 |volume=81 |issue=4 |pages=545–579 |pmid=17038201 |doi=10.1017/S1464793106007111 }}</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 |s2cid=45809019 }}</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|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 |s2cid=29778484 }}</ref>

====Heart type and features====

[[File:Didactic model of an avian heart-FMVZ USP-13.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>Whittow, G. (2000). Sturkie's Avian Physiology/ edited by G. Causey Whittow. San Diego : Academic Press, 2000.</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.

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">Hoagstrom, C.W. (2002). "Vertebrate Circulation". ''Magill's Encyclopedia of Science: Animal Life''. Vol 1, pp. 217–219. Pasadena, California, Salem Press.</ref>

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.<ref name=Hill>Hill, Richard W. (2012) Animal Physiology/ Richard W. Hill, Gordon A. Wyse, Margaret Anderson. Third Edition pp. 647–678. Sinauer Associates, Sunderland, MA</ref> 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.

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.

[[File:Bird blink-edit.jpg|thumb|left|upright=1.35|The [[nictitating membrane]] as it covers the eye of a [[masked lapwing]]]]

The [[nervous system]] is large relative to the bird's size.<ref name = "Gill"/> The most developed part of the brain 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|pages=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 |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 |url-status=dead}}</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 [[tubenoses]].<ref>{{Cite journal|last=Lequette |first=Benoit |date=1 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 |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 |author2=Verheyden |author3=Jouventin |url-status=dead|jstor=1368131 }}</ref> The 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=PM |last3=Das |first3=D |last4=Degrip |first4=WJ |last5=Bowmaker |first5=JK |last6=Hunt |first6=DM |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 name="OlssonLind2018">{{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|issn=1045-2249|doi=10.1093/beheco/arx133|s2cid=90704358|doi-access=free}}</ref>

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 reflective 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 |s2cid=4356193 }}</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"/>

Eyes 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|s2cid=15836436 }}</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|s2cid=44351032 }}</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 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 }}</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 |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===

[[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 }}</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 |issue=1 |jstor=4081839|last2=Parkes|first2=K.C.|doi=10.2307/4081839}}</ref> Moulting is annual in most species, although [[Glossary of bird terms#cite note-32|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|isbn=0-12-552455-2|publisher=Burgess Publishing Co}}</ref>

A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.<ref name="debeeretal">de Beer SJ, Lockwood GM, Raijmakers JHFS, Raijmakers JMH, Scott WA, Oschadleus HD, Underhill LG (2001). "[http://safring.adu.org.za/downloads/ringers-manual.pdf SAFRING Bird Ringing Manual] {{Webarchive|url=https://web.archive.org/web/20171019032817/http://safring.adu.org.za/downloads/ringers-manual.pdf |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 |url-status=dead|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 |url-status=dead |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 |s2cid=26584982 }}</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]]

Feathers require maintenance and birds 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 |url-status=dead}}</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>

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 ([[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, probably due to limited resources and the absence of 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|s2cid=86511951 }}</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. |year=2000 |title=Anatomy and histochemistry of flight muscles in a wing-propelled diving bird, the Atlantic Puffin, ''Fratercula arctica'' |journal=Journal of Morphology |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 |last2=Meyers |first2=RA}}</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 |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 |doi=10.2307/4087761 |last2=McNeil |first2=Raymond |last3=Leduc |first3=Alain |url-status=dead|jstor=4087761 }}</ref>

[[File:BirdBeaksA.svg|thumb|upright|right|alt= Illustration of the heads of 16 types of birds with different shapes and sizes of beak|Feeding adaptations in beaks]]

{{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|url=http://sora.unm.edu/sites/default/files/journals/condor/v097n01/p0057-p0067.pdf|journal=Condor|volume=97|issue=1|pages=57–67|doi=10.2307/1368983|jstor=1368983}}</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=Bio One|doi=10.1525/auk.2010.09155|lay-url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2990222/|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 name="Attenborough2">{{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}}  (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=N Reid |year=2006 |url-status=dead |archive-url=https://web.archive.org/web/20110315005353/http://lwa.gov.au/files/products/land-water-and-wool/pf061365/pf061365.pdf |archive-date=15 March 2011 }}</ref> Combined, insectivorous birds eat 400–500&nbsp;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 }}</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}}</ref> [[Loon]]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 |s2cid=84979098 }}</ref> [[Geese]] and [[dabbling duck]]s are primarily grazers.

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 |pages=486 |doi=10.2307/3677021 |jstor=3677021 }}</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|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 |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.|hdl=10261/47141 |hdl-access=free }}</ref>

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|url-status=dead}}</ref> Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.<ref>{{Cite journal|title=The role of hyperthermia in the water economy of desert birds|journal= Physiol. Biochem. Zool.|volume=72|year=1999|pages=87–100|doi=10.1086/316640|pmid=9882607|last1=Tieleman|first1=B.I.|last2=Williams|first2=JB|issue=1|s2cid= 18920080|url= https://pure.rug.nl/ws/files/62402849/The_Role_of_Hyperthermia_in_the_Water_Economy_of_Desert_Birds.pdf}}</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|s2cid=2757501|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 their daily congregations 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|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}}</ref>

===Feather care===

Feathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, [[ectoparasitic]] feather mites and [[Bird louse|bird lice]].<ref>{{cite journal|title=The alterations of plumage of parasitic origin|first1=Principato|last1=Mario|first2=Lisi|last2=Federica|first3=Moretta|last3=Iolanda|first4=Samra|last4=Nada|first5=Puccetti|last5=Francesco|journal=Italian Journal of Animal Science|volume=4|issue=3|pages=296–299|year=2005|doi=10.4081/ijas.2005.296|s2cid=84770232|doi-access=free}}</ref> The physical condition of feathers are maintained by {{Birdgloss|preening}} often with the application of secretions from the {{Birdgloss|preen gland}}. Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as [[Anting (bird activity)|anting]] in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.<ref>{{cite journal|journal=The Auk |volume=121|issue=4|pages=1262–1268| year=2004| doi=10.1642/0004-8038(2004)121[1262:BAFAOA]2.0.CO;2| title=Bactericidal and fungicidal activity of ant chemicals on feather parasites: an evaluation of anting behavior as a method of self-medication in songbirds| first1=Hannah C. |last1=Revis|first2=Deborah A. |last2=Waller}}</ref><ref>{{cite journal|journal=The Open Ornithology Journal|year=2010|volume=3|pages=41–71|doi=10.2174/1874453201003010041|title=How Birds Combat Ectoparasites|first1=Dale H.|last1=Clayton|first2=Jennifer A.H.|last2=Koop|first3=Christopher W.|last3=Harbison|first4=Brett R.|last4=Moyer|first5=Sarah E.|last5=Bush|doi-access=free}}</ref>

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=MW |last4=Tang |first4=S |last5=Dekinga |first5=A |last6=Hulsman |first6=K |pmc=1690512}}  (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 |url=http://jeb.biologists.org/cgi/reprint/199/1/57 }}</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}}</ref> [[Seabird]]s also undertake long migrations, the longest annual migration being those of [[sooty shearwater]]s, which nest in [[New Zealand]] and [[Chile]] and spend the northern summer feeding in the North Pacific off Japan, [[Alaska]] and [[California]], an annual round trip of {{convert|64000|km|mi|-2|abbr=on}}.<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=GA |last9=Foley |first9=DG|pmc=1568927|first10=BA|last10=Block|first11=DP|last11=Costa|display-authors=1 |bibcode=2006PNAS..10312799S }}</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=JR |last3=Phillips |first3=RA |last4=Afanasyev |first4=V |last5=Briggs |first5=DR|bibcode=2005Sci...307..249C |s2cid=28990783 }}</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 |url-status=dead}}</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}}</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|s2cid=82259620 }}</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}}</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 (biology)|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 |volume=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 |url=http://jeb.biologists.org/cgi/reprint/30/3/370 }}</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|last=Mouritsen |first=Henrik |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 |url=http://jeb.biologists.org/cgi/content/full/204/22/3855 |author2=L|doi=10.1242/jeb.204.22.3855 }}</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|last=Deutschlander |first=Mark E. |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 |url=http://jeb.biologists.org/cgi/reprint/202/8/891 |author2=P |author3=B|doi=10.1242/jeb.202.8.891 }}</ref>

{{Listen|filename=Troglodytes aedon - House Wren - XC79974.ogg|title=Bird song|description=Song of the [[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.}}

[[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.]]

Birds [[Animal communication|communicate]] using primarily 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 |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 |url-status=dead|jstor=1368675 }}</ref> Variation in plumage also allows for the identification of birds, particularly between species.

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 |url=http://www.marineornithology.org/PDF/29_1/29_1_6.pdf}}</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 |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 |doi=10.1080/08927014.1994.9522988 |last1=Genevois |first1=F. |year=1994 |last2=Bretagnolle |first2=V. |title=Male Blue Petrels reveal their body mass when calling |journal=Ethology Ecology and Evolution |volume=6 |issue=3 |pages=377–383 |url=http://ejour-fup.unifi.it/index.php/eee/article/view/667/613 |url-status=dead |archive-url=https://web.archive.org/web/20071224040649/http://ejour-fup.unifi.it/index.php/eee/article/view/667/613 |archive-date=24 December 2007}}</ref> bond formation, the claiming and maintenance of territories,<ref name = "Gill"/> 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|s2cid=45578269 }}</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 |s2cid=42276496 }}</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|issn=0010-5422|doi=10.1650/0010-5422(2001)103[0143:DWDEIF]2.0.CO;2}}</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}}</ref>

 

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[[File:Caribbean Flamingo2 (Phoenicopterus ruber) (0424) - Relic38.jpg|thumb|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. 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=Michael |year=2000 |title=Sleep-Vigilance Trade-off in Gadwall during the Winter Period |journal=The Condor |volume=102 |issue=2 |pages=307–313 |url=http://sora.unm.edu/sites/default/files/journals/condor/v102n02/p0307-p0313.pdf |archive-url=https://web.archive.org/web/20041227194439/http://sora.unm.edu/sites/default/files/journals/condor/v102n02/p0307-p0313.pdf |archive-date=27 December 2004 |doi=10.1650/0010-5422(2000)102[0307:SVTOIG]2.0.CO;2|last2=Tamisier |first2=Alain |last3=Cézilly |first3=Frank|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}}</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|s2cid=1736369 }}</ref>

 

 

 

 

[[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}}</ref>]]

Ninety-five 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|s2cid=84735736}}</ref> Monogamy allows for both [[paternal care]] and [[Parental investment|biparental care]], which is especially important for species in which females require males' assistance for successful brood-rearing.<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|s2cid=84258620}}</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.|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|last2=Buschhaus|first2=Nancy|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|s2cid=85745432}}</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>

 

 

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 arena)|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.|s2cid=26038182}}</ref> [[Courtship feeding]], [[Billing (birds)|billing]] and {{Birdgloss|allopreening}} are commonly performed between partners, generally after the birds have paired and mated.<ref name="Attenborough">{{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>