Editing Bird (section)

===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>