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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 has become 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 .
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 and 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.
Chicken can mean a chick, and this was historically the meaning of the word chicken, as in William Shakespeare's play Macbeth, where Macduff laments the death of "all my pretty chickens and their dam".Shakespeare, William, Macbeth, Act 4 Scene 3, lines 217–229. The usage is preserved in placenames such as the Hen and Chicken Islands. In older sources, and still often in trade and scientific contexts, chickens as a species are described as common fowl or domestic fowl.
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 . In the wild, they scratch at the soil to search for seeds, insects, and animals as large as , small snakes, and young mouse. 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 herd, and Egg incubation 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". (Reprinted from Zeitschrift für Psychologie, 1922, 88:225–252.) 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, , 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 to indicate that a Predation is approaching from the air or on the ground.
Sperm transfer occurs by 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 (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.
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 Broodiness". 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.
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.
Middle Eastern chicken remains go back to a little earlier than 2000 BC in Syria.The Cambridge History of Food, 2000, Cambridge University Press, Vol. 1, pp. 496–499 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.
Breeding increased under the Roman Empire and reduced in the Middle Ages. DNA 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.
Chickens farmed primarily for eggs are called layer hens. The UK alone consumes more than 34 million eggs per day. Hens of some breeds can produce over 300 eggs per year; the highest authenticated rate of egg laying is 371 eggs in 364 days.
The chicken was the first bird species to have its genome sequenced. At 1.21 Gigabase, the chicken genome is similarly sized compared to other birds, but smaller than nearly all mammals: the human genome is 3.2 Gigabase. The final gene set contained 26,640 genes (including noncoding genes and ), with a total of 19,119 protein-coding genes, a similar number to the human genome. 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. Scientists Find Chickens Retain Ancient Ability to Grow Teeth Ammu Kannampilly, ABC News, February 27, 2006. Retrieved October 1, 2007.
Dispersal
Austronesia
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 may have been introduced during pre-Columbian times to South America via Polynesians seafarers, but this is disputed.
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. Chicken bones from the Arauco Peninsula in Zona Sur 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.
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. Chickens were widespread throughout southern Central Asia by the 4th century BC.
Africa
Chickens reached Egypt via the Middle East for purposes of cockfighting about 1400 BC and became widely bred in Egypt around 300 BC. Three possible routes of introduction into Africa around the early first millennium AD could have been through the Egyptian Nile River Valley, the East Africa Roman-Greek or Indian trade, or from Carthage and the Berbers, across the Sahara Desert. The earliest known remains are from Mali, Nubia, East Coast, and South Africa and date back to the middle of the first millennium AD.
Diseases
Chickens are susceptible both to Parasitism such as , and to Poultry disease caused by such as bacteria and . 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.
Viral diseases include avian influenza.
Use by humans
Farming
Chickens are common and widespread domestic animals, with a total population of 23.7 billion . 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 towards sentient animals. 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. Chickens farmed for meat are called . Broiler breeds typically take less than six weeks to reach slaughter size, some weeks longer for free range and organic broilers.
(2011). 9780440423102, Jim Pattison Group. ISBN 9780440423102 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. 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 Forced molting 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. Chickens are one of the most efficient sources of foods for many different purposes.
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 and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities.
Cockfighting
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 civilisation from 2500 to 2100 BC. In the process of domestication, chickens were apparently kept initially for cockfighting, and only later used for food.
In science
Chickens have long been used as 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.
In culture, folklore, and religion
Chickens are featured 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.
Roosters are sometimes used for divination, a practice called alectryomancy. This involves the sacrifice of a sacred rooster, often during a ritual cockfight, used as a form of communication with the gods. In Gabriel García Márquez's Nobel-Prize-winning 1967 novel One Hundred Years of Solitude, cockfighting is outlawed in the town of Macondo after the patriarch of the Buendia family murders his cockfighting rival and is haunted by the man's ghost. have been made at least since The Knickerbocker published one in 1847. The Knickerbocker, or The New York Monthly, March 1847, p. 283. Chickens have been featured in art in farmyard scenes such as Adriaen van Utrecht's 1646 Turkeys and Chickens and Walter Osborne's 1885 Feeding the Chickens. The nursery rhyme "Cock a doodle doo", its chorus line imitating the cockerel's call, was published in Mother Goose's Melody in 1765.
The 2000 animated adventure film comedy film Chicken Run, directed by Peter Lord and Nick Park, featured Anthropomorphism chickens with many chicken jokes.
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