Editing Chicken (section)

== Reproduction and life-cycle ==

To initiate courting, some roosters may dance in a circle around or near a hen (a circle dance), often lowering the wing which is closest to the hen.<ref name="grandin69">{{cite book |title=Animals in Translation |last1=Grandin |first1=Temple |author-link1=Temple Grandin |last2=Johnson |first2=Catherine |year=2005 |publisher=[[Scribner's]] |location=New York City |isbn=978-0-7432-4769-6 |pages=[https://archive.org/details/animalsintransla00gran/page/69 69–71] |url-access=registration |url=https://archive.org/details/animalsintransla00gran/page/69 }}</ref> The dance triggers a response in the hen<ref name="grandin69" /> and when she responds to his call, the rooster may mount the hen and proceed with the mating. Mating typically involves a sequence in which the male approaches the female and performs a waltzing display. If the female is unreceptive, she runs off; otherwise, she crouches, and the male mounts, treading with both feet on her back. After copulation the male does a tail-bending display.<ref>{{cite journal |last1=Cheng |first1=Kimberly M. |last2=Burns |first2=Jeffrey T. |title=Dominance Relationship and Mating Behavior of Domestic Cocks: A Model to Study Mate-Guarding and Sperm Competition in Birds |journal=[[Ornithological Applications|The Condor]] |date=August 1988 |volume=90 |issue=3 |pages=697–704 |doi=10.2307/1368360 |jstor=1368360 }}</ref>

Sperm transfer occurs by [[cloaca]]l contact between the male and female, in an action called the 'cloacal kiss'.<ref name="Briskie1997">{{cite journal |last=Briskie |first=J. V. |author2=R. Montgomerie |year=1997 |title=Sexual Selection and the Intromittent Organ of Birds |journal=Journal of Avian Biology |volume=28 |issue=1 |pages=73–86 |doi=10.2307/3677097 |jstor=3677097}}</ref> As with all birds, [[reproduction]] is controlled by a [[neuroendocrine cell|neuroendocrine]] system,<ref>{{Cite journal |last1=Dufour |first1=Sylvie |last2=Quérat |first2=Bruno |last3=Tostivint |first3=Hervé |last4=Pasqualini |first4=Catherine |last5=Vaudry |first5=Hubert |last6=Rousseau |first6=Karine |date=April 2020 |title=Origin and Evolution of the Neuroendocrine Control of Reproduction in Vertebrates, With Special Focus on Genome and Gene Duplications |url=https://journals.physiology.org/doi/full/10.1152/physrev.00009.2019 |journal=Physiological Reviews |volume=100 |issue=2 |pages=869–943 |doi=10.1152/physrev.00009.2019 |pmid=31625459 |issn=0031-9333}}</ref> the [[Gonadotropin-Releasing Hormone-I neuron]]s in the [[hypothalamus]]. Reproductive hormones including [[estrogen]], [[progesterone]], and [[gonadotropin]]s ([[luteinizing hormone]] and [[follicle-stimulating hormone]]) initiate and maintain sexual maturation changes. Reproduction declines with age, thought to be due to a decline in GnRH-I-N.<ref name="Bain-et-al-2016">{{cite journal |last1=Bain |first1=M. M. |last2=Nys |first2=Y. |last3=Dunn |first3=I.C. |title=Increasing persistency in lay and stabilising egg quality in longer laying cycles. What are the challenges? |journal=[[British Poultry Science]] |publisher=[[Taylor & Francis]] |volume=57 |issue=3 |date=May 3, 2016 |doi=10.1080/00071668.2016.1161727 |pages=330–338 |pmid=26982003 |pmc=4940894 |s2cid=17842329 |doi-access=free }}</ref>

[[File:Newly-hatched chickens.jpg|thumb|Newly hatched chicks]]

Hens often try to lay in nests that already contain eggs and sometimes move eggs from neighbouring nests into their own. A flock thus uses only a few preferred locations, rather than having a different nest for every bird.<ref>{{cite journal |last1=Sherwin |first1=C.M. |last2=Nicol |first2=C.J. |year=1993 |title=Factors influencing floor-laying by hens in modified cages |journal=[[Applied Animal Behaviour Science]] |volume=36 |issue=2–3 |pages=211–222 |doi=10.1016/0168-1591(93)90011-d}}</ref> Under natural conditions, most birds lay only until a [[Clutch (eggs)|clutch]] is complete; they then incubate all the eggs. This is called "going [[Broodiness|broody]]". The hen sits on the nest, fluffing up or pecking defensively if disturbed. She rarely leaves the nest until the eggs have hatched.<ref name="Puff-up-Feathers">{{cite web |title=Why Do Chickens Puff up Their Feathers? I 4 Reasons Explained|date= August 8, 2020|url= https://chickenandchicksinfo.com/why-do-chickens-puff-up-their-feathers/ |access-date=June 16, 2021 |archive-date=June 18, 2021 |archive-url=https://web.archive.org/web/20210618091939/https://chickenandchicksinfo.com/why-do-chickens-puff-up-their-feathers/ |url-status=live}}</ref>

Eggs of chickens from the high-altitude region of [[Tibet]] have special physiological adaptations that result in a higher hatching rate in low oxygen environments. When eggs are placed in a hypoxic environment, chicken embryos from these populations express much more [[hemoglobin]] than embryos from other chicken populations. This hemoglobin has a greater affinity for oxygen, binding oxygen more readily.<ref>{{Cite journal |last1=Zhang |first1=H. |last2=Wang |first2=X.T. |last3=Chamba |first3=Y. |last4=Ling |first4=Y.|last5=Wu|first5=C.X. |date=October 2008|title=Influences of Hypoxia on Hatching Performance in Chickens with Different Genetic Adaptation to High Altitude |journal=Poultry Science |volume=87 |issue=10 |pages=2112–2116 |doi=10.3382/ps.2008-00122 |pmid=18809874 |doi-access=free}}</ref>

Fertile chicken eggs hatch at the end of the incubation period, about 21 days; the chick uses its [[egg tooth]] to break out of the shell.<ref name="grandin69"/> Hens remain on the nest for about two days after the first chick hatches; during this time the newly hatched chicks feed by absorbing the internal [[yolk sac]].<ref>{{cite journal |last1=Ali |first1=A. |last2=Cheng |first2=K.M. |year=1985 |title=Early egg production in genetically blind (rc/rc) chickens in comparison with sighted (Rc+/rc) controls |journal=Poultry Science |volume=64 |issue=5 |pages=789–794 |doi=10.3382/ps.0640789 |pmid=4001066 |doi-access=free }}</ref> The hen guards her chicks and broods them to keep them warm. She leads them to food and water and calls them towards food. The chicks [[Imprinting (psychology)|imprint]] on the hen and subsequently follow her continually. She continues to care for them until they are several weeks old.<ref name="Edgar Held Jones Troisi 2016 p. 2">{{cite journal |last1=Edgar |first1=Joanne |last2=Held |first2=Suzanne |last3=Jones |first3=Charlotte |last4=Troisi |first4=Camille |title=Influences of Maternal Care on Chicken Welfare |journal=Animals |volume=6 |issue=1 |date=January 5, 2016 |pmid=26742081 |pmc=4730119 |doi=10.3390/ani6010002 |doi-access=free |page=2}}</ref>

Inbreeding of White Leghorn chickens tends to cause [[inbreeding depression]] expressed as reduced egg number and delayed sexual maturity.<ref>{{cite journal |last1=Sewalem |first1=A. |last2=Johansson |first2=K. |last3=Wilhelmson |first3=M. |last4=Lillpers |first4=K. |title=Inbreeding and inbreeding depression on reproduction and production traits of White Leghorn lines selected for egg production traits |journal=British Poultry Science |volume=40 |issue=2 |date=1999 |doi=10.1080/00071669987601 |pages=203–208|pmid=10465386 }}</ref> Strongly inbred Langshan chickens display obvious inbreeding depression in reproduction, particularly for traits such as age when the first egg is laid and egg number.<ref>{{cite journal |last1=Xue |first1=Qian |last2=Li |first2=Guohui |last3=Cao |first3=Yuxia |last4=Yin |first4=Jianmei |last5=Zhu |first5=Yunfen |last6=Zhang |first6=Huiyong |last7=Zhou |first7=Chenghao |last8=Shen |first8=Haiyu |last9=Dou |first9=Xinhong |last10=Su |first10=Yijun |last11=Wang |first11=Kehua |last12=Zou |first12=Jianmin |last13=Han |first13=Wei |title=Identification of genes involved in inbreeding depression of reproduction in Langshan chickens |journal=Animal Bioscience |volume=34 |issue=6 |date=June 1, 2021 |issn=2765-0189 |pmid=33152217 |pmc=8100482 |doi=10.5713/ajas.20.0248 |pages=975–984}}</ref>