Chicken

The chicks imprint on the hen and subsequently follow her continually. 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. Hens often try to lay in nests that already contain eggs and sometimes move eggs from neighbouring nests into their own. As with all birds, reproduction is controlled by a neuroendocrine system, the Gonadotropin-Releasing Hormone-I neurons in the hypothalamus.

Farming

Descendants of those domestications have spread throughout the world in several waves for at least the last 2,000 years. Chickens belonging to the same age cohort and sex are often kept together in industrial production settings. A flock usually includes one dominant adult male, a few subdominant males, and two or more females that are carefully watched over by the dominant male. Chicks are born covered in down, but they mature quickly, becoming fully feathered after four to five weeks. Fertilized embryos develop quickly, and chicks hatch approximately 21 days later. There is some debate about what the chicken’s scientific name should eaub.info/bd/ be.

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The hen sits on the nest, fluffing up or pecking defensively if disturbed. A flock thus uses only a few preferred locations, rather than having a different nest for every bird. Reproduction declines with age, thought to be due to a decline in GnRH-I-N.

Genetic sequencing of chicken bones from archaeological sites in Europe revealed that in the High Middle Ages chickens became less aggressive and began to lay eggs earlier in the breeding season. The possibility that domestic chickens were in the Americas before Western contact is debated by researchers, but blue-egged chickens, found only in the Americas and Asia, suggest an Asian origin for early American chickens. Analysis of the most popular commercial breed shows that the White Leghorn breed possesses a mosaic of divergent ancestries inherited from different subspecies of red junglefowl.

• The chicken was the first bird species to have its genome sequenced.
• Chickens are primarily kept for their meat and eggs, though they are also kept as pets.
• 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.
• Inbreeding of White Leghorn chickens tends to cause inbreeding depression expressed as reduced egg number and delayed sexual maturity.
• The first pictures of chickens in Europe are found on Corinthian pottery of the 7th century BC.

In situations where one adult bird challenges another—which happens most often when a new bird is introduced into the flock—fights involving males risk injury and death more often than fights involving females. In groups of male chicks, however, fights for dominance may continue into adulthood. The pecking order is established within groups of female chicks by the 10th week of life. The time between ovulation and egg-laying is approximately 23–26 hours. In 2006, scientists researching the ancestry of birds switched on a chicken recessive gene, talpid2, and found that the embryo jaws initiated formation of teeth, like those found in ancient bird fossils. Large numbers of embryos can be provided commercially; fertilized eggs can easily be opened and used to observe the developing embryo.