Chicken

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. 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.

Domestication

Domesticated chickens freely interbreed with populations of red junglefowl. 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. Only hens that could no longer produce enough eggs were killed and sold for meat. Only in the early 20th century, however, did chicken meat and eggs become mass-production commodities.

Individual chickens dominate others, establishing a pecking order; dominant individuals take priority for access to food and nest sites. Chickens are primarily kept for their meat and eggs, though they are also kept as pets. 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.

Farming

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. Archaeological evidence appeared to support domestic chickens in Southeast Asia well before 6000 BC, China by 6000 BC and India by 2000 BC.

• The chicks imprint on the hen and subsequently follow her continually.
• An early study proposed that a single domestication event of the red junglefowl in present-day Thailand gave rise to the modern chicken.
• By the mid-20th century, however, meat production had outstripped egg production as a specialized industry.
• Only hens that could no longer produce enough eggs were killed and sold for meat.
• Individual chickens dominate others, establishing a pecking order; dominant individuals take priority for access to food and nest sites.

Many immature males (cockerels) are castrated (usually chemically, with hormones that cause atrophying of the testicles) to become meat birds (capons). The market for chicken meat has grown dramatically since then, with worldwide exports reaching nearly 12.5 million metric tons (about 13.8 million tons) by the early 21st century. By the mid-20th century, however, meat production had outstripped egg production as a specialized industry.

For most of that period, chickens were a common part of the livestock complement of farms and ranches throughout Eurasia and Africa. Chicken domestication likely occurred more than once in Southeast Asia and possibly India over the most recent 7,400 years, and the first domestications may have been for religious reasons or for the raising of fighting birds. Each flock of chickens develops a social hierarchy that determines access to food, nesting sites, mates, and other resources. At about six months, males produce viable sperm, and females produce viable eggs. Despite the chicken’s close relationship with the red jungle fowl, there is evidence that the gray jungle fowl (G. sonneratii) of southern India and other jungle fowl species, also members of Gallus, may have contributed to the bird’s ancestry.

Farming

For instance, many important discoveries in limb development have been made using chicken embryos, such as the discovery of the apical ectodermal ridge and the zone of polarizing activity. Equally important, embryologists can carry out experiments on such embryos, close the egg again and study the effects later in development. A cockfight is a contest held in a ring called a cockpit between two cocks. Certain breeds, such as silkies and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities. Chickens are social, inquisitive, intelligent birds, and many people find their behaviour entertaining. This involves complete withdrawal of food (and sometimes water) for 7–14 days or sufficiently khela 88 long to cause a body weight loss of 25 to 35%, or up to 28 days under experimental conditions.