Chicken

For the culinary use of chickens, see Chicken as food. For other uses, see Chicken (disambiguation) and Chooks (disambiguation).

"Rooster" and "Roosters" redirect here. For other uses, see Rooster (disambiguation).

"Cockerel" redirects here. For the Fabergé egg, see Cockerel (Fabergé egg).

Chicken
A rooster (left) and hen (right) perching on a roost
Conservation status
Domesticated
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Galliformes
Family:	Phasianidae
Genus:	Gallus
Species:	G. domesticus
Binomial name
Gallus domesticus (Linnaeus, 1758)
Chicken distribution

The chicken (Gallus domesticus) is a domesticated species that arose from the red junglefowl, originally from Southeast Asia. They have hybridized with other wild species of junglefowl (the grey junglefowl, Ceylon junglefowl, and green junglefowl). Rooster and cock are terms for adult male birds, and a younger male may be called a cockerel. A male that has been castrated is a capon. An adult female bird is called a hen, and a sexually immature female is called a pullet. Humans keep chickens primarily as a source of food (consuming both their meat and eggs) or as pets. Traditionally, they were also bred for cockfighting, which is still practiced in some places. Chickens domesticated for meat are broilers, and for eggs, they are layers.

Chickens are common and widespread domestic animals, with a total population of 23.7 billion as of 2018, up from more than 19 billion in 2011. There are more chickens in the world than any other bird. There are numerous cultural references to chickens—in myth, folklore, and religion, as well as in language and literature.

Genetic studies have pointed to multiple maternal origin theories within South Asia, Southeast Asia, and East Asia, but the clade found in the Americas, Europe, the Middle East, and Africa originated from the Indian subcontinent. From ancient India, the chicken spread to the Eastern Mediterranean. They appear in ancient Egypt in the mid-15th century BC, with the "bird that gives birth every day" having come from the land between Syria and Shinar, Babylonia, according to the annals of Thutmose III. They are known in ancient Greece from the 5th century BC.

Nomenclature

Terms for chickens include

• Biddy: a newly hatched chicken
• Capon: a castrated or neutered male chicken
• Chick: a young chicken
• Chook /tʃʊk/: a chicken (Australia/New Zealand, informal)
• Cock: an adult male chicken
• Cockerel: a young male chicken less than a year old
• Dunghill fowl: a chicken with mixed parentage from different domestic varieties.
• 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 male chicken, especially in North America. Originated in the 18th century as a euphemism to avoid the sexual connotation of the British English cock.
• Yardbird: a chicken (southern United States, dialectal)

Chicken may mean a chick (see for example Hen and Chicken Islands). The term originally meant an immature or young bird. In older sources, and still often in trade and scientific contexts, chicken as a species are typically referred to as common fowl or domestic fowl.

In Australian vernacular English the word chook provides the generic term for the species (e.g. "a cooked chook" or "she keeps chooks"), which enables chicken to retain its original sense of a young or recently hatched bird. Chick is then rarely used to mean chicken, but is mainly used in Merriam-Webster's "Sense 1b" viz. the young of any bird.

Biology

Comb of male

Comb of female, generally smaller

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, according to Guinness World Records.

Adult chickens 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. Collectively, these and other fleshy protuberances on the head and throat are called caruncles. Both the adult male and female have wattles and combs, but in most breeds, these are more prominent in males. A 'muff' or 'beard' is a mutation found in several chicken breeds that causes extra feathering under the chicken's face, giving the appearance of a beard.

Behaviour

Social behaviour

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. Removing hens or roosters from a flock disruptsthis social order until a new pecking order is established. Adding hens, especially younger birds, to an existing flock can lead to fighting and injury.

Chickens are capable of ganging up on and killing a weak or inexperienced predator, such as a young fox.

Vocalizations

"Cockadoodledoo" and "Cocka-doodle-doo" redirect here. For the nursery rhyme, see Cock a doodle doo.

Normal length crowing (with audio)

A rooster's crowing is a loud and sometimes shrill call, serving as a territorial signal to other roosters. However, roosters may also crow 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.

Courtship

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. 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.

More specifically, mating typically involves the following sequence:

1. Male approaching the hen
2. Male pre-copulatory waltzing
3. Male waltzing
4. Female crouching (receptive posture) or stepping aside or running away (if unwilling to copulate)
5. Male mounting
6. Male treading with both feet on hen's back
7. Male tail bending (following successful copulation)

Nesting and laying behaviour

Hens often try to lay in nests that already contain eggs and sometimes move eggs from neighbouring nests into their own. The result of this is that a flock uses only a few preferred locations, rather than having a different nest for every bird. Hens often express a preference to lay in the same location. Two or more hens may try to share the same nest at the same time. If the nest is small or one of the hens is particularly determined, chickens may try to lay on top of each other. There is evidence that individual hens prefer to be either solitary or gregarious nesters.

Broodiness

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. The hen rarely leaves the nest until the eggs have hatched.

Hatching and early life

Two Lavender Orpington chicks.

A group of chicks

Fertile chicken eggs hatch at the end of the incubation period, about 21 days. Development of the chick starts only when incubation begins, so all chicks hatch within a day or two of each other, despite perhaps being laid over a period of two weeks or so. Before hatching, the hen can hear the chicks peeping inside the eggs and will gently cluck to stimulate them to break out of their shells. The chick begins by pipping: pecking a breathing hole with its egg tooth towards the blunt end of the egg, usually on the upper side. The chick then rests for some hours, absorbing the remaining egg yolk and withdrawing the blood supply from the membrane beneath the shell (used earlier for breathing through the shell). The chick then enlarges the hole, gradually turning round as it goes, and eventually severing the blunt end of the shell completely to make a lid. The chick crawls out of the remaining shell, and the wet down dries out in the warmth of the nest.

Hens usually remain on the nest for about two days after the first chick hatches, and during this time the newly hatched chicks feed by absorbing the internal yolk sac. Some breeds sometimes start eating cracked eggs, which can become habitual. Hens fiercely guard their chicks and brood them when necessary to keep them warm, at first often returning to the nest at night. She leads them to food and water and will call them toward edible items but seldom feeds them directly. She continues to care for them until they are several weeks old.

Reproduction

Sperm transfer occurs by cloacal contact between the male and female, in a maneuver known as the 'cloacal kiss'. As with birds in general, reproduction is controlled by a neuroendocrine system, the Gonadotropin-Releasing Hormone-I neurons in the hypothalamus. Locally to the reproductive system, reproductive hormones such as estrogen, progesterone, gonadotropins (luteinizing hormone and follicle-stimulating hormone) initiate and maintain sexual maturation changes. Over time there is reproductive decline, thought to be due to GnRH-I-N decline. Because there is significant inter-individual variability in egg-producing duration, it is believed to be possible to breed for further extended useful lifetime in egg-layers.

Embryology

(Video) Earliest gestation stages and blood circulation of a chicken embryo

Chicken embryos have long been used as model organisms to study developing embryos. Large numbers of embryos can be provided by commercial chicken farmers who sell fertilized eggs which can be easily 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 effect later on. For instance, many important discoveries in the area of limb development have been made using chicken embryos, such as the discovery of the apical ectodermal ridge and the zone of polarizing activity by John W. Saunders.

In 2006, scientists researching the ancestry of birds "turned on" a chicken recessive gene, talpid2, and found that the embryo jaws initiated formation of teeth, like those found in ancient bird fossils. John Fallon, the overseer of the project, stated that chickens have "...retained the ability to make teeth, under certain conditions... ."

Genetics and genomics

Mottle gene

Given its eminent role in farming, meat production, but also research, the house chicken was the first bird genome to be sequenced. At 1.21 Gb, the chicken genome is similarly sized compared to other birds, but smaller than nearly all mammals, such as the human genome (of 3.2 Gb). The final gene set contained 26,640 genes (including noncoding genes and pseudogenes), with a total of 19,119 protein-coding genes in annotation release 103 (2017), a similar number of protein-coding genes as in the human genome.

Physiology

Populations of chickens from high-altitude regions like 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 also has a greater affinity for oxygen, allowing hemoglobin to bind to oxygen more readily.

Pinopsins were originally discovered in the chicken pineal gland.

Although all avians appear to have lost TLR9, artificial immunity against bacterial pathogens has been induced in neonatal chicks by Taghavi et al. 2008 using tailored oligodeoxynucleotides.

Origin and dispersal

Origin

Galliformes, the order of bird that chickens belong to, is directly linked to the survival of birds when all other dinosaurs went extinct. Water or ground-dwelling fowl, similar to modern partridges, survived the Cretaceous–Paleogene extinction event that killed all tree-dwelling birds and dinosaurs. Some of these evolved into the modern galliformes, of which domesticated chickens are a main model. They are descended primarily from the red junglefowl (Gallus gallus) and are scientifically classified as the same species. As such, domesticated chickens can and do freely interbreed with populations of red junglefowl. Subsequent hybridization of the domestic chicken with grey junglefowl, Sri Lankan junglefowl and green junglefowl occurred; a gene for yellow skin, for instance, was incorporated into domestic birds through hybridization with the grey junglefowl (G. sonneratii). In a study published in 2020, it was found that chickens shared between 71 and 79% of their genome with red junglefowl, with the period of domestication dated to 8,000 years ago.

Domestication

Further information: 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. 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 this predisposition for prolific reproduction of the red junglefowl when exposed to large amounts of food.

Exactly when and where the chicken was domesticated remains controversial. Genomic studies estimate 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 supports domestic chickens in Southeast Asia well before 6000 BC, China by 6000 BC and India by 2000 BC. 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.

Dispersal

Map showing the prehistoric introduction of domesticated chickens into Oceania from the Philippines via the Neolithic Austronesian expansion (starting at c. 4000 BP), inferred from genetic markers on ancient and modern chicken DNA (Thomson et al., 2014)

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. These chickens might have been introduced during pre-Columbian times to South America via Polynesian seafarers, but evidence for this is still putative.

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. 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. The first pictures of chickens in Europe are found on Corinthian pottery of the 7th century BC.

Chickens reached Europe c. 100 BC. Breeding increased under the Roman Empire and reduced by the Middle Ages. 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.

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.

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. 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. However, further study of the same bones cast doubt on the findings.

Diseases

Main article: Poultry disease

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.

Use by humans

Farming

Main articles: Poultry farming and Chicken as food

More than 50 billion chickens are reared annually as a source of meat and eggs. In the United States alone, more than 8 billion chickens are slaughtered each year for meat, and more than 300 million chickens are reared for egg production. 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. 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. Advocates of intensive farming say that their highly efficient systems save land and food resources owing to increased productivity, and that the animals are looked after in state-of-the-art environmentally controlled facilities.

Reared for meat

A commercial chicken house with open sides raising broiler pullets for meat

Chickens farmed for meat are called broilers. Chickens will naturally live for six or more years, but broiler breeds typically take less than six weeks to reach slaughter size. A free range or organic broiler will usually be slaughtered at about 14 weeks of age.

Reared for eggs

Main article: Egg as food

Chickens farmed primarily for eggs are called layer hens. In total, the UK alone consumes more than 34 million eggs per day. Some hen breeds can produce over 300 eggs per year, with the highest authenticated rate of egg laying being 371 eggs in 364 days. After 12 months of laying, the commercial hen's egg-laying ability starts to decline 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.

In the UK and Europe, laying hens are then slaughtered and used in processed foods or sold as 'soup hens'. 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 or sufficiently long to cause a body weight loss of 25 to 35%, or up to 28 days under experimental conditions. 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.

As pets

Keeping chickens as pets became increasingly popular in the 2000s among urban and suburban residents. 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.

Chickens are social, inquisitive, intelligent birds, and many people find their behaviour entertaining. Certain breeds, such as silkies and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities. Many people feed chickens in part with kitchen food scraps.

Cockfighting

Main article: Cockfight

A cockfight in Tamil Nadu, India, 2011

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. The activity seems to have been practised in the Indus Valley from 2500–2100 BC. In the process of domestication, chickens were apparently kept initially for cockfighting, and only later used for food.

Artificial incubation

Incubation can occur artificially in machines that provide the correct, controlled environment for the developing chick. The average incubation period for chickens is 21 days, but the duration depends on the temperature and humidity in the incubator. Temperature regulation is the most critical factor for a successful hatch. Variations of more than 1 °C (1.8 °F) from the optimum temperature of 37.5 °C (99.5 °F) reduce hatch rates. Humidity is important because the rate at which eggs lose water by evaporation depends on the ambient relative humidity. Evaporation can be assessed by candling, to view the size of the air sac, or by measuring weight loss. Relative humidity should be increased to around 70% in the last three days of incubation to keep the membrane around the hatching chick from drying out after the chick cracks the shell. Lower humidity is usual in the first 18 days to ensure adequate evaporation.

In culture, folklore, and religion

Main article: Cultural references to chickens

Chickens feature widely in myth, folklore, religion, language, and literature. Chickens are a sacred animal in many cultures and are deeply embedded in many belief systems and religious worship practices.

Roosters are sometimes used for divination, a practice called alectryomancy. This involves the sacrifice of a sacred rooster, often takes place during a ritual cockfight, used as a form of communication with the gods.

• 
  Etruscan askos in the form of a rooster, 4th century B.C.
• 
  Rooster and hen, Đông Hồ folk woodcut, Vietnam
• 
  Joseph Crawhall III, Spanish Cock and Snail, c. 1900
• 
  Wooden chicken mask, Bali, late 20th century
• 
  Carved and painted wooden tribal statue of a cock fight, Yoruba, West Africa, c. 2000

External links

• Data related to Gallus gallus domesticus at Wikispecies