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Chicken

From Wikipedia, the free encyclopedia

Hatching and early life
Fertile chicken eggs hatch at the end of the incubation period, about 21 days. Development of the chick starts only when incubation begins, so all chicks hatch within a day or two of each other, despite perhaps being laid over a period of two weeks or so. Before hatching, the hen can hear the chicks peeping inside the eggs, and will gently cluck to stimulate them to break out of their shells. The chick begins by "pipping"; pecking a breathing hole with its egg tooth towards the blunt end of the egg, usually on the upper side. The chick then rests for some hours, absorbing the remaining egg yolk and withdrawing the blood supply from the membrane beneath the shell (used earlier for breathing through the shell). The chick then enlarges the hole, gradually turning round as it goes, and eventually severing the blunt end of the shell completely to make a lid. The chick crawls out of the remaining shell, and the wet down dries out in the warmth of the nest.

 My addition  (Adaptations to gallus gallus have allowed for a greater success rate in hatchability. When eggs are placed in a hypoxic environment, chicken embryos express a much greater amount of hemoglobin than other chicken species. [107][108])

Hens usually remain on the nest for about two days after the first chick hatches, and during this time the newly hatched chicks feed by absorbing the internal yolk sac. Some breeds sometimes start eating cracked eggs, which can become habitual. Hens fiercely guard their chicks, and brood them when necessary to keep them warm, at first often returning to the nest at night. She leads them to food and water and will call them toward edible items, but seldom feeds them directly. She continues to care for them until they are several weeks old.

Defensive behaviour

Chickens may occasionally gang up on a weak or inexperienced predator. At least one credible report exists of a young fox killed by hens. A group of hens have been recorded in attacking a hawk that had entered their coop.

References

107.       Gou X, Li N, Lian L, Yan D, Zhang H, Wei Z, Wu C. Hypoxic adaptations of hemoglobin in Tibetan chick embryo: high oxygen-affinity mutation and selective expression. Comp Biochem Physiol B Biochem Mol Biol. 2007 Jun;147(2):147-55. doi:                 10.1016/j.cbpb.2006.11.031. Epub 2007 Jan 24. PMID: 17360214.

108.       H. Zhang, X.T. Wang, Y. Chamba, Y. Ling, C.X. Wu, Influences of Hypoxia on Hatching Performance in Chickens with Different Genetic Adaptation to High Altitude, Poultry Science, Volume 87, Issue 10, 2008, Pages 2112-2116, ISSN 0032-5791