Chicken

Chicken may mean a chick (as in Hen and Chicken Islands).^[15] In older sources, and still often in trade and scientific contexts, chickens as a species are described as common fowl or domestic fowl.^[16]

Description

Comb of male

Comb of female, generally smaller

Chickens are relatively large

Chickens are omnivores.^[20] In the wild, they scratch at the soil to search for seeds, insects, and animals as large as lizards, small snakes,^[21] and young mice.^[22] A chicken may live for 5–10 years, depending on the breed.^[23] The world's oldest known chicken lived for 16 years.^[24]

Chickens are

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.[31] The dance triggers a response in the hen^[31] 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.^[32]

Sperm transfer occurs by

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.^[35] 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.^[36]

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.^[37]

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.^[31] 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.^[38] The hen guards her chicks and brood 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.^[39]

Origin

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.^[40] Chickens are descended primarily from the red junglefowl (Gallus gallus) and are scientifically classified as the same species.^[41] Domesticated chickens freely interbreed with populations of red junglefowl.^[41] The domestic chicken has subsequently hybridised with grey junglefowl, Sri Lankan junglefowl and green junglefowl;^[42] a gene for yellow skin, for instance, was incorporated into domestic birds from the grey junglefowl (G. sonneratii).^[43] It is estimated that chickens share between 71 and 79% of their genome with red junglefowl.^[42]

Domestication

According to one early study, a single domestication event of the red junglefowl in present-day Thailand gave rise to the modern chicken with minor transitions separating the modern breeds.^[44] 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.^[45] 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.^[46]

Exactly when and where the chicken was domesticated remains controversial. Genomic studies estimate that the chicken was domesticated 8,000 years ago^[42] in Southeast Asia and spread to China and India 2,000 to 3,000 years later. Archaeological evidence supports domestic chickens in Southeast Asia well before 6000 BC, China by 6000 BC and India by 2000 BC.^[42][47][48] A landmark 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 southwestern China, northern Thailand and Myanmar. 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 subspecies of red junglefowl.^[49][50][51]

Dispersal

Oceania

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. A lack of data from Thailand, Russia, the Indian subcontinent, Southeast Asia and Sub-Saharan Africa makes it difficult to lay out a clear map of the spread of chickens in these areas; better description and genetic analysis of local breeds threatened by extinction may also help with research into this area.^[57] Chicken bones from the Arauco Peninsula in south-central Chile were radiocarbon dated as pre-Columbian, and DNA analysis suggested they were related to prehistoric populations in Polynesia.^[58][59] However, further study of the same bones cast doubt on the findings.^[60][61]

Eurasia

Chicken remains have been difficult to date, given the small and fragile bird bones; this may account for discrepancies in dates given by different sources. Archaeological evidence is supplemented by mentions in historical texts from the last few centuries BC, and by depictions in prehistoric artworks, such as across Central Asia.^[62] Chickens were widespread throughout southern Central Asia by the 4th century BC.^[62]

Middle Eastern chicken remains go back to a little earlier than 2000 BC in

Breeding increased under the Roman Empire and reduced in the Middle Ages.^[57] 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.^[66]

Africa

Chickens reached Egypt via the Middle East for purposes of

Chickens are susceptible both to parasites such as mites, and to diseases caused by pathogens such as bacteria and viruses. The parasite Dermanyssus gallinae feeds on blood, causing irritation and reducing egg production, and acts as a vector for bacterial diseases such as salmonellosis and spirochaetosis.^[67] Viral diseases include avian influenza.^[68]

Use by humans

Farming

Chickens are common and widespread domestic animals, with a total population of 23.7 billion as of 2018^[update].

Chickens farmed primarily for eggs are called layer hens. The UK alone consumes more than 34 million eggs per day.^[78] Hens of some breeds can produce over 300 eggs per year; the highest authenticated rate of egg laying is 371 eggs in 364 days.^[79] After 12 months of laying, the commercial hen's egg-laying ability declines to the point where the flock is commercially unviable. Hens, particularly from battery cage systems, are sometimes infirm or have lost a significant amount of their feathers, and their life expectancy has been reduced from around seven years to less than two years.^[80] In the UK and Europe, laying hens are then slaughtered and used in processed foods, or sold as 'soup hens'.^[80] In some other countries, flocks are sometimes force moulted rather than being slaughtered to re-invigorate egg-laying. This involves complete withdrawal of food (and sometimes water) for 7–14 days^[81] or sufficiently long to cause a body weight loss of 25 to 35%,^[82] or up to 28 days under experimental conditions.^[83] This stimulates the hen to lose her feathers but also re-invigorates egg-production. Some flocks may be force-moulted several times. In 2003, more than 75% of all flocks were moulted in the US.^[84]

As pets

Keeping chickens as pets became increasingly popular in the 2000s^[85] among urban and suburban residents.^[86] Many people obtain chickens for their egg production but often name them and treat them as any other pet like cats or dogs. Chickens provide companionship and have individual personalities. While many do not cuddle much, they will eat from one's hand, jump onto one's lap, respond to and follow their handlers, as well as show affection.^[87][88] Chickens are social, inquisitive, intelligent^[89] birds, and many people find their behaviour entertaining.^[90] Certain breeds, such as silkies and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities.^[91]

Cockfighting

A

Chickens have long been used as model organisms to study developing embryos. Large numbers of embryos can be provided commercially; fertilized eggs can easily be opened and used to observe the developing embryo. Equally important, embryologists can carry out experiments on such embryos, close the egg again and study the effects later in development. 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.^[95]

The chicken was the first bird species to have its

Chickens feature widely in folklore, religion, literature, and popular culture. The chicken is a sacred animal in many cultures and deeply embedded in belief systems and religious practices.^[100] Roosters are sometimes used for