Editing Bird (section)

==Anatomy and physiology==
{{Main|Bird anatomy|}}
{{See also|Egg tooth}}
[[File:Birdmorphology.svg|thumb|upright=1.35|right|External anatomy of a bird (example: [[yellow-wattled lapwing]]):

# Beak
# Head
# Iris
# Pupil
# Mantle
# Lesser [[Covert (feather)|coverts]]
# Scapulars
# Median coverts
# Tertials
# Rump
# Primaries
# [[Cloaca#Birds|Vent]]
# Thigh
# Tibio-tarsal articulation
# Tarsus
# Foot
# Tibia
# Belly
# Flanks
# Breast
# Throat
# Wattle
# Eyestripe

]]

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

===Skeletal system===
{{Main|Bird_anatomy#Skeletal_system}}

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

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

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

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

Birds are solely [[Gonochorism|gonochoric]],<ref>{{cite book |doi=10.1007/978-4-431-56609-0_14 |quote-page=290 |quote=Mammals and birds are solely gonochoristic. In mammals, the sex is determined by the genetic male heterogametic system (XX–XY). In birds, the female heterogametic ssytem (ZZ-ZW) is used. |chapter=Genetic Control of Sex Determination and Differentiation in Fish |title=Reproductive and Developmental Strategies |series=Diversity and Commonality in Animals |date=2018 |last1=Matsuda |first1=Masaru |pages=289–306 |isbn=978-4-431-56607-6 }}</ref> meaning they have two sexes: either [[female]] or [[male]]. The sex of birds 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"/> A complex system of [[disassortative mating]] with two morphs is involved in the [[white-throated sparrow]] ''Zonotrichia albicollis'', where white- and tan-browed morphs of opposite sex pair, making it appear as if four sexes were involved since any individual is compatible with only a fourth of the population.<ref>{{Cite journal |last1=Tuttle |first1=Elaina M. |last2=Bergland |first2=Alan O. |last3=Korody |first3=Marisa L. |last4=Brewer |first4=Michael S. |last5=Newhouse |first5=Daniel J. |last6=Minx |first6=Patrick |last7=Stager |first7=Maria |last8=Betuel |first8=Adam |last9=Cheviron |first9=Zachary A. |last10=Warren |first10=Wesley C. |last11=Gonser |first11=Rusty A. |last12=Balakrishnan |first12=Christopher N. |date=2016 |title=Divergence and Functional Degradation of a Sex Chromosome-like Supergene |journal=Current Biology |language=en |volume=26 |issue=3 |pages=344–350 |doi=10.1016/j.cub.2015.11.069 |pmc=4747794 |pmid=26804558|bibcode=2016CBio...26..344T }}</ref>

In nearly all species of birds, an individual's sex is determined at fertilisation. However, one 2007 study claimed to demonstrate [[temperature-dependent sex determination]] among the [[Australian brushturkey]], for which higher temperatures during incubation resulted in a higher female-to-male [[sex ratio]].<ref>{{Cite journal|last=Göth|first=Anne|title=Incubation temperatures and sex ratios in Australian brush-turkey (''Alectura lathami'') mounds |journal=Austral Ecology|year=2007|volume=32|issue=4|pages=278–285 |doi=10.1111/j.1442-9993.2007.01709.x|bibcode=2007AusEc..32..378G }}</ref> This, however, was later proven to not be the case. These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality.<ref>{{cite journal|title=Temperature-dependent sex ratio in a bird|volume=1|issue=1|date=March 2005|pages=31–33 |last1=Göth |first1=A |last2=Booth |first2=DT |journal=Biology Letters |pmc=1629050 |pmid=17148121 |doi=10.1098/rsbl.2004.0247}}</ref>

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

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

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

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

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

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

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

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

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

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

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

===Feathers, plumage, and scales===
{{Main|Feather|Flight feather|Down feather}}
[[File:African Scops owl.jpg|alt=Owl with eyes closed in front of similarly coloured tree trunk partly obscured by green leaves|thumb|left|The [[disruptively patterned]] plumage of the [[African scops owl]] allows it to blend in with its surroundings.]]

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

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

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

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

===Flight===
{{Main|Bird flight|Flightless birds}}
[[File:Restless flycatcher04.jpg|left|alt=Black bird with white chest in flight with wings facing down and tail fanned and down pointing| thumb|[[Restless flycatcher]] in the downstroke of flapping flight]]
Most birds can [[Flying and gliding animals|fly]], which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb ([[Bird wing|wing]]) that serves as an [[airfoil|aerofoil]].<ref name="Gill"/>

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