Chicken
The chicken is a domesticated form of the red junglefowl, originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is one of the most common and widespread domesticated animals in the world. Chickens are primarily kept for their meat and eggs, though they are also kept as pets.
As of 2023, the global chicken population exceeds 26.5 billion, with more than 50 billion birds produced annually for consumption. Specialized breeds such as broilers and laying hens have been developed for meat and egg production, respectively. A hen bred for laying can produce over 300 eggs per year. Chickens are social animals with complex vocalizations and behaviors, and feature in folklore, religion, and literature across many societies. Their economic importance makes them a central component of global animal husbandry.
Nomenclature
Terms for chickens include:- Biddy: a chicken, or a newly hatched chicken
- Capon: a castrated or neutered male chicken
- Chick: a young chicken
- Chook : a chicken
- Cock: a fertile adult male chicken
- Cockerel: a young male chicken
- Hen: an adult female chicken
- Pullet: a young female chicken less than a year old. In the poultry industry, a pullet is a sexually immature chicken less than 22 weeks of age.
- Rooster: a fertile adult male chicken, especially in North America. Originated in the 18th century, possibly as a euphemism to avoid the sexual connotation of the word cock.
- Yardbird: a chicken
Description
Chickens are relatively large birds, active by day. The body is round, the legs are unfeathered in most breeds, and the wings are short. Wild junglefowl can fly, whereas domestic chickens and their flight muscles are too heavy to allow them to fly more than a short distance. Size and coloration vary widely between breeds. Newly hatched chicks of both modern and heritage varieties weigh the same, about. Modern varieties however grow much faster; by day 35 a Ross 708 broiler may weigh as against the of a heritage chicken of the same age.Adult chickens of both sexes have a fleshy crest on their heads called a comb or cockscomb, and hanging flaps of skin on either side under their beaks called wattles; combs and wattles are more prominent in males. Some breeds have a mutation that causes extra feathering under the face, giving the appearance of a beard.
Chickens are omnivores. In the wild, they scratch at the soil to search for seeds, insects, and animals as large as lizards, small snakes, and young mice. A chicken may live for 5–10 years, depending on the breed. The world's oldest known chicken lived for 16 years.
Chickens are gregarious, living in flocks, and incubate eggs and raise young communally. Individual chickens dominate others, establishing a pecking order; dominant individuals take priority for access to food and nest sites. The concept of dominance, involving pecking, was described in female chickens by Thorleif Schjelderup-Ebbe in 1921 as the "pecking order". Male chickens tend to leap and use their claws in conflicts. Chickens are capable of mobbing and killing a weak or inexperienced predator, such as a young fox. Chickens have been thought of primarily as providers of food, but their cognition, emotions, and sociality are comparable with other birds and mammals.
A male's crowing is a loud and sometimes shrill call, serving as a territorial signal to other males, and in response to sudden disturbances within their surroundings. Hens cluck loudly after laying an egg and to call their chicks. Chickens give different warning calls to indicate that a predator is approaching from the air or on the ground.
Reproduction and life-cycle
To initiate courting, some roosters may dance in a circle around or near a hen, often lowering the wing which is closest to the hen. The dance triggers a response in the hen 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.Sperm transfer occurs by cloacal contact between the male and female, in an action called the 'cloacal kiss'. As with all birds, reproduction is controlled by a neuroendocrine system, the Gonadotropin-Releasing Hormone-I neurons in the hypothalamus. Reproductive hormones including estrogen, progesterone, and gonadotropins initiate and maintain sexual maturation changes. Reproduction declines with age, thought to be due to a decline in GnRH-I-N.
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. Under natural conditions, most birds lay only until a clutch is complete; they then incubate all the eggs. This is called "going broody". The hen sits on the nest, fluffing up or pecking defensively if disturbed. She rarely leaves the nest until the eggs have hatched.
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.
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. 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 guards her chicks and broods them to keep them warm. She leads them to food and water and calls them towards food. The chicks imprint on the hen and subsequently follow her continually. She continues to care for them until they are several weeks old.
Inbreeding of White Leghorn chickens tends to cause inbreeding depression expressed as reduced egg number and delayed sexual maturity. 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.
Origin
Phylogeny
Water or ground-dwelling fowl similar to modern partridges, in the Galliformes, the order of bird that chickens belong to, survived the Cretaceous–Paleogene extinction event that killed all tree-dwelling birds and their dinosaur relatives. Chickens are descended primarily from the red junglefowl and are scientifically classified as the same species. Domesticated chickens freely interbreed with populations of red junglefowl. The domestic chicken has subsequently hybridised with grey junglefowl, Sri Lankan junglefowl and green junglefowl; a gene for yellow skin, for instance, was incorporated into domestic birds from the grey junglefowl. It is estimated that chickens share between 71 and 79% of their genome with red junglefowl.Domestication
The red junglefowl is well adapted to take advantage of the vast quantities of seed produced during the end of the multi-decade bamboo seeding cycle, to boost its own reproduction. In domesticating the chicken, humans took advantage of the red junglefowl's ability to reproduce prolifically when exposed to a surge in its food supply.Exactly when and where the chicken was domesticated was controversial. Genomic studies estimated that the chicken was domesticated 8,000 years ago in Southeast Asia and spread to China and India 2,000 to 3,000 years later. Archaeological evidence appeared to support domestic chickens in Southeast Asia well before 6000 BC, China by 6000 BC and India by 2000 BC. An early study proposed that a single domestication event of the red junglefowl in present-day Thailand gave rise to the modern chicken. A 2020 Nature study that fully sequenced 863 chickens across the world suggests that all domestic chickens originate from a single domestication event of red junglefowl whose present-day distribution is predominantly in Southeast Asia. These domesticated chickens spread across Southeast and South Asia where they interbred with local wild species of junglefowl, forming genetically and geographically distinct groups. 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.
Re-examination of bones from over 600 sites, and dating of those from 23 sites, identified the earliest probable chicken bones as from central Thailand, at Ban Non Wat, some 3,250 years ago. The paleo-anatomist Joris Peters and the bioarchaeologist Greger Larson state that this coincided with the growing of rice, and propose that wild jungle fowl were attracted to eat the rice seeds, nested nearby, and became domesticated. Skeletons of birds in the Gallus genus were used as grave goods at the site, confirming domestication.