Chickens are common and widespread domestic animals, with a total population of 23.7 billion as of 2018[update], and an annual production of more than 50 billion birds. A hen bred for laying can produce over 300 eggs per year. There are numerous
cultural references to chickens in folklore, religion, and literature.
Nomenclature
Terms for chickens include:
Biddy: a chicken, or a newly hatched chicken[1][2]
Pullet: a young female chicken less than a year old.[9] In the poultry industry, a pullet is a sexually immature chicken less than 22 weeks of age.[10]
Rooster: a fertile adult male chicken, especially in North America. Originated in the 18th century as a euphemism to avoid the sexual connotation of the British English cock.[11][12][13]
Yardbird: a chicken (southern United States, dialectal)[14]
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]
Chickens are relatively large
birds,
active by day. The body is round, the legs are unfeathered in most breeds, and the wings are short.[17] Wild
junglefowl can
fly; chickens and their
flight muscles are too heavy to allow them to fly more than a short distance.[18] Size and coloration vary widely between breeds.[17] 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.[19]
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
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".[25][26] Male chickens tend to leap and use their claws in conflicts.[27] Chickens are capable of mobbing and killing a weak or inexperienced predator, such as a young fox.[28]
A male's crowing is a loud and sometimes shrill call, serving as a territorial signal to other males,[29] 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.[30]
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.[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]
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 and dispersal
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]
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]
Middle Eastern chicken remains go back to a little earlier than 2000 BC in
Syria. They reached Egypt for purposes of
cockfighting about 1400 BC and became widely bred in Egypt around 300 BC.[57] 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.[58] The first pictures of chickens in Europe are found on
Corinthianpottery of the 7th century BC.[59][60]
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.[61]
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.[57]
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.[62][63] However, further study of the same bones cast doubt on the findings.[64][65]
Chickens are common and widespread domestic animals, with a total population of 23.7 billion as of 2018[update].[68] More than 50 billion chickens are reared annually as a source of meat and eggs.[69] In the United States alone, more than 8 billion chickens are slaughtered each year for meat,[70] and more than 300 million chickens are reared for egg production.[71] The vast majority of poultry is raised in
factory farms. According to the
Worldwatch Institute, 74% of the world's poultry meat and 68% of eggs are produced this way.[72] An alternative to intensive poultry farming is
free-range farming. Friction between these two main methods has led to long-term issues of
ethical consumerism. Opponents of
intensive farming argue that it harms the environment, creates human health risks and is inhumane.[73] Advocates of intensive farming say that their efficient systems save land and food resources owing to increased productivity, and that the animals are looked after in a controlled environment.[74] Chickens farmed for meat are called
broilers. Broiler breeds typically take less than six weeks to reach slaughter size,[75] some weeks longer for
free range and
organic broilers.[76]
Chickens farmed primarily for eggs are called layer hens. The UK alone consumes more than 34 million eggs per day.[77] Hens of some breeds can produce over 300 eggs per year; the highest authenticated rate of egg laying is 371 eggs in 364 days.[78] 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.[79] In the UK and Europe, laying hens are then slaughtered and used in processed foods, or sold as 'soup hens'.[79] 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[80] or sufficiently long to cause a body weight loss of 25 to 35%,[81] or up to 28 days under experimental conditions.[82] 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.[83]
As pets
Keeping chickens as pets became increasingly popular in the 2000s[84] among urban and suburban residents.[85] 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.[86][87] Chickens are social, inquisitive, intelligent[88] birds, and many people find their behaviour entertaining.[89] Certain breeds, such as
silkies and many
bantam varieties, are generally docile and are often recommended as good pets around children with disabilities.[90]
A
cockfight is a contest held in a ring called a cockpit between two cocks. Cockfighting is outlawed in many countries as involving
cruelty to animals.[91] The activity seems to have been practised in the
Indus Valley civilisation from 2500–2100 BC.[92] In the process of domestication, chickens were apparently kept initially for cockfighting, and only later used for food.[93]
In science
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.[94]
The chicken was the first bird species to have its
genome sequenced.[95] At 1.21 Gb, the chicken genome is similarly sized compared to other birds, but smaller than nearly all mammals: the
human genome is 3.2 Gb.[96] The final gene set contained 26,640 genes (including noncoding genes and
pseudogenes), with a total of 19,119 protein-coding genes, a similar number to the human genome.[97] 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.[98]
^"Chick". Cambridge Dictionary.
Archived from the original on September 7, 2015.
^"Chook". Cambridge Dictionary.
Archived from the original on September 7, 2015. Retrieved March 4, 2021.
^"Cock". Cambridge Dictionary.
Archived from the original on September 7, 2015. Retrieved March 4, 2021.
^"Hen". Cambridge Dictionary.
Archived from the original on September 7, 2015. Retrieved March 4, 2021.
^Cockerel. Dictionary.reference.com.
Archived from the original on March 7, 2016. Retrieved August 29, 2010.
^"Hen noun". Merriam-Webster. Retrieved February 2, 2024.
^Pullet. Dictionary.reference.com.
Archived from the original on November 9, 2010. Retrieved August 29, 2010.
^"Overview of the Poultry Industry"(PDF). Overview of the Poultry Industry. Missouri Department of Elementary and Secondary Education. p. 8.
Archived(PDF) from the original on October 23, 2020.
^Perrin, P. G. (1955). "'Pecking order' 1927–54". American Speech. 30 (4): 265–268.
doi:
10.2307/453561.
JSTOR453561.
^Schjelderup-Ebbe, T. (1975). "Contributions to the social psychology of the domestic chicken [Schleidt M., Schleidt, W. M., translators]". In Schein, M. W. (ed.). Social Hierarchy and Dominance. Benchmark Papers in Animal Behavior. Vol. 3. Stroudsburg, Pennsylvania: Dowden, Hutchinson and Ross. pp. 35–49. (Reprinted from Zeitschrift für Psychologie, 1922, 88:225-252.)
^Evans, Christopher S.; Evans, Linda; Marler, Peter (July 1993). "On the meaning of alarm calls: functional reference in an avian vocal system". Animal Behaviour. 46 (1): 23–38.
doi:
10.1006/anbe.1993.1158.
S2CID53165305.
^Cheng, Kimberly M.; Burns, Jeffrey T. (August 1988). "Dominance Relationship and Mating Behavior of Domestic Cocks: A Model to Study Mate-Guarding and Sperm Competition in Birds". The Condor. 90 (3): 697–704.
doi:
10.2307/1368360.
JSTOR1368360.
^Briskie, J. V.; R. Montgomerie (1997). "Sexual Selection and the Intromittent Organ of Birds". Journal of Avian Biology. 28 (1): 73–86.
doi:
10.2307/3677097.
JSTOR3677097.
^Fumihito, A.; Miyake, T.; Sumi, S.; Takada, M.; Ohno, S.; Kondo, N. (December 20, 1994), "One subspecies of the red junglefowl (Gallus gallus gallus) suffices as the matriarchic ancestor of all domestic breeds", PNAS, 91 (26): 12505–12509,
Bibcode:
1994PNAS...9112505F,
doi:10.1073/pnas.91.26.12505,
PMC45467,
PMID7809067
^King, Rick (February 24, 2009),
"Rat Attack", Nova and National Geographic Television,
archived from the original on August 23, 2017, retrieved August 25, 2017
^King, Rick (February 1, 2009),
"Plant vs. Predator", NOVA,
archived from the original on August 21, 2017, retrieved August 25, 2017
^Liu, Yi-Ping; Wu, Gui-Sheng; Yao, Yong-Gang; Miao, Yong-Wang; Luikart, Gordon; Baig, Mumtaz; et al. (January 2006). "Multiple maternal origins of chickens: Out of the Asian jungles". Molecular Phylogenetics and Evolution. 38 (1): 12–19.
doi:
10.1016/j.ympev.2005.09.014.
PMID16275023.
^Zeder, Melinda A.; Emshwiller, Eve; Smith, Bruce D.; Bradley, Daniel G. (March 2006). "Documenting domestication: the intersection of genetics and archaeology". Trends in Genetics. 22 (3): 139–155.
doi:
10.1016/j.tig.2006.01.007.
PMID16458995.
^Karayanis, Dean; Karayanis, Catherine (March 13, 2019).
Regional Greek Cooking. Hippocrene Books. p. 176.
ISBN9780781811460.
Archived from the original on September 13, 2016. Retrieved March 13, 2019 – via Google Books.
^Brown, Marley (September–October 2017).
"Fast Food". Archaeology. 70 (5): 18.
ISSN0003-8113.
Archived from the original on July 25, 2019. Retrieved July 25, 2019.
^Gregory, T. Ryan (September 2005). "Synergy between sequence and size in Large-scale genomics". Nature Reviews Genetics. 6 (9): 699–708.
doi:
10.1038/nrg1674.
PMID16151375.
S2CID24237594.
^The Knickerbocker, or The New York Monthly, March 1847, p. 283.
^Kellogg, Diane M. (May 22, 2020).
"Chickens in Art History". Painting World Magazine. Archived from
the original on February 2, 2024. Retrieved February 2, 2024.
^Opie, Iona; Opie, Peter (1997) [1951]. The Oxford Dictionary of Nursery Rhymes (2nd ed.). Oxford University Press. p. 128.