The chicken (Gallus gallus domesticus) is a domesticated subspecies of the red

junglefowl (Gallus gallus), originally native to Southeast Asia. It was first

domesticated around 8,000 years ago and is now 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.[1]

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
prominently in folklore, religion, and literature across many societies. Their
economic importance makes them a central component of global animal husbandry and
agriculture.

Nomenclature
Terms for chickens include:

Biddy: a chicken, or a newly hatched chicken[2][3]

Capon: a castrated or neutered male chicken[a]
Chick: a young chicken[4]
Chook /tʃʊk/: a chicken (Australia/New Zealand, informal)[5]
Cock: a fertile adult male chicken[6][7]
Cockerel: a young male chicken[8]
Hen: an adult female chicken[9]
Pullet: a young female chicken less than a year old.[10] In the poultry industry, a
pullet is a sexually immature chicken less than 22 weeks of age.[11]
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.[12][13][14]
Yardbird: a chicken (southern United States, dialectal)[15]
Chicken can mean a chick, and this was historically the meaning of the word
chicken,[16] as in William Shakespeare's play Macbeth, where Macduff laments the
death of "all my pretty chickens and their dam".[17] The usage is preserved in
placenames such as the Hen and Chicken Islands.[18] In older sources, and still
often in trade and scientific contexts, chickens as a species are described as
common fowl or domestic fowl.[19]

Description

Comb and wattles of male

Comb of female, generally smaller

Chickens are relatively large birds, active by day. The body is round, the legs are
unfeathered in most breeds, and the wings are short.[20] Wild junglefowl can fly;
chickens and their flight muscles are too heavy to allow them to fly more than a
short distance.[21] Size and coloration vary widely between breeds.[20] Newly-
hatched chicks of both modern and heritage varieties weigh the same, about 37 g
(1.3 oz). Modern varieties however grow much faster; by day 35 a Ross 708 broiler
may weigh 1.8 kg (4.0 lb) as against the 1.05 kg (2.3 lb) of a heritage chicken of
the same age.[22]

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.[23]

Chickens are omnivores.[24] In the wild, they scratch at the soil to search for
seeds, insects, and animals as large as lizards, small snakes,[25] and young mice.
[26] A chicken may live for 5–10 years, depending on the breed.[27] The world's
oldest known chicken lived for 16 years.[28]

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".[29][30] Male chickens tend to leap
and use their claws in conflicts.[31] Chickens are capable of mobbing and killing a
weak or inexperienced predator, such as a young fox.[32]

Duration: 6 seconds.0:06
Crowing (with audio)
A male's crowing is a loud and sometimes shrill call, serving as a territorial
signal to other males,[33] 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.[34]

Reproduction and life-cycle

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.[35] The dance
triggers a response in the hen[35] 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.[36]

Sperm transfer occurs by cloacal contact between the male and female, in an action
called the 'cloacal kiss'.[37] As with all birds, reproduction is controlled by a
neuroendocrine system,[38] the Gonadotropin-Releasing Hormone-I neurons in the
hypothalamus. Reproductive hormones including estrogen, progesterone, and
gonadotropins (luteinizing hormone and follicle-stimulating hormone) initiate and
maintain sexual maturation changes. Reproduction declines with age, thought to be
due to a decline in GnRH-I-N.[39]

Newly hatched chicks

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.[40] 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.[41]

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.[42]

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.[35] 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.[43] 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.[44]

Inbreeding of White Leghorn chickens tends to cause inbreeding depression expressed

as reduced egg number and delayed sexual maturity.[45] 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.[46]

Origin
Phylogeny

Red junglefowl, the wild ancestor of the chicken

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.[47]
Chickens are descended primarily from the red junglefowl (Gallus gallus) and are
scientifically classified as the same species.[48] Domesticated chickens freely
interbreed with populations of red junglefowl.[48] The domestic chicken has
subsequently hybridised with grey junglefowl, Sri Lankan junglefowl and green
junglefowl;[49] a gene for yellow skin, for instance, was incorporated into
domestic birds from the grey junglefowl (G. sonneratii).[50] It is estimated that
chickens share between 71 and 79% of their genome with red junglefowl.[49]

Domestication
Further information: Domestication

Chicken domestication and dispersal;[49] possibility of early arrival in

Americas[51][52]
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.[53] 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.[54] 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.[55]

Exactly when and where the chicken was domesticated remains controversial. Genomic
studies estimate that the chicken was domesticated 8,000 years ago[49] 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.[49][56][57] 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.[58][59][60]

Dispersal
Austronesia

Prehistoric introduction of domesticated chickens into Oceania from the Philippines

via Neolithic Austronesian expansion (starting at c. 4000 BP), inferred from
genetic markers on ancient and modern chicken DNA (Thomson et al., 2014)[61]
A word for the domestic chicken (*manuk) is part of the reconstructed Proto-
Austronesian language, indicating they were domesticated by the Austronesian
peoples since ancient times. Chickens, together with dogs and pigs, were carried
throughout the entire range of the prehistoric Austronesian maritime migrations to
Island Southeast Asia, Micronesia, Island Melanesia, Polynesia, and Madagascar,
starting from at least 3000 BC from Taiwan.[61][62][63][64] These chickens may have
been introduced during pre-Columbian times to South America via Polynesian
seafarers, but this is disputed.[65]

Americas
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.[66] 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.[51]
[52] However, further study of the same bones cast doubt on the findings.[67][68]

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.[69] Chickens were widespread throughout southern Central Asia by the
4th century BC.[69]

Middle Eastern chicken remains go back to a little earlier than 2000 BC in Syria.
[66] Phoenicians spread chickens along the Mediterranean coasts as far as Iberia.
During the Hellenistic period (4th–2nd centuries BC), in the southern Levant,
chickens began to be widely domesticated for food.[70] The first pictures of
chickens in Europe are found on Corinthian pottery of the 7th century BC.[71][72]

Breeding increased under the Roman Empire and reduced in the Middle Ages.[66]
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.[73]

Africa
Chickens reached Egypt via the Middle East for purposes of cockfighting about 1400
BC and became widely bred in Egypt around 300 BC.[66] Three possible routes of
introduction into Africa around the early first millennium AD could have been
through the Egyptian Nile Valley, the East Africa Roman-Greek or Indian trade, or
from Carthage and the Berbers, across the Sahara. The earliest known remains are
from Mali, Nubia, East Coast, and South Africa and date back to the middle of the
first millennium AD.[66]