Chickens are common and widespread domestic animals, with a total population of 26.5 billion as of 2023[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 numerouscultural references to chickens in folklore, religion, and literature.
Nomenclature
Terms for chickens include:
Biddy: a chicken, or a newly hatched chicken[2][3]
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 wordcock.[12][13][14]
Yardbird: a chicken (southern United States, dialectal)[15]
Chicken can mean achick, and this was historically the meaning of the word chicken,[16] as inWilliam Shakespeare's playMacbeth, whereMacduff laments the death of "all my pretty chickens and their dam".[17] The usage is preserved in placenames such as theHen and Chicken Islands.[18] In older sources, and still often in trade and scientific contexts, chickens as a species are described ascommon fowl ordomestic fowl.[19]
Chickens are relatively largebirds,active by day. The body is round, the legs are unfeathered in most breeds, and the wings are short.[20] Wildjunglefowl canfly; chickens and theirflight 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 708broiler 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 calledwattles; combs and wattles aremore prominent in males. Some breeds have amutation that causes extra feathering under the face, giving the appearance of a beard.[23]
Chickens areomnivores.[24] In the wild, they scratch at the soil to search for seeds, insects, and animals as large aslizards, small snakes,[25] and youngmice.[26] A chicken may live for 5–10 years, depending on thebreed.[27] The world's oldest known chicken lived for 16 years.[28]
Chickens aregregarious, living inflocks, andincubate eggs and raise young communally. Individual chickens dominate others, establishing apecking order; dominant individuals take priority for access to food and nest sites. The concept of dominance, involving pecking, was described in female chickens byThorleif 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]
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 anegg and to call their chicks. Chickens give differentwarning calls to indicate that apredator 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]
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 aclutch is complete; they then incubate all the eggs. This is called "goingbroody". 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 ofTibet 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 morehemoglobin 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 itsegg 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 internalyolk 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 chicksimprint 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 causeinbreeding 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]
Water or ground-dwelling fowl similar to modernpartridges, in theGalliformes, theorder of bird that chickens belong to, survived theCretaceous–Paleogene extinction event that killed all tree-dwelling birds and theirdinosaur relatives.[47] Chickens are descended primarily from thered 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 withgrey junglefowl,Sri Lankan junglefowl andgreen 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]
Chicken domestication and dispersal;[49] possibility of early arrival in Americas[51][52]
According to one early study, a single domestication event of thered junglefowl in present-dayThailand 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 themulti-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 intoOceania from thePhilippines viaNeolithicAustronesian expansion (starting at c. 4000BP), inferred from genetic markers on ancient and modern chicken DNA (Thomsonet al., 2014)[61]
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 byextinction may also help with research into this area.[66] Chicken bones from theArauco Peninsula insouth-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 inSyria.[66] Phoenicians spread chickens along the Mediterranean coasts as far as Iberia. During theHellenistic period (4th–2nd centuries BC), in the southernLevant, chickens began to be widely domesticated for food.[70] The first pictures of chickens in Europe are found onCorinthianpottery of the 7th century BC.[71][72]
Breeding increased under theRoman Empire and reduced in theMiddle Ages.[66]Genetic sequencing of chicken bones from archaeological sites in Europe revealed that in theHigh 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 ofcockfighting 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 EgyptianNile Valley, the East Africa Roman-Greek or Indian trade, or from Carthage and the Berbers, across theSahara. The earliest known remains are fromMali,Nubia, East Coast, andSouth Africa and date back to the middle of the first millennium AD.[66]
Chickens are common and widespread domestic animals, with a total population of 23.7 billion as of 2018[update].[76] More than 50 billion chickens are reared annually as a source of meat and eggs.[77] In the United States alone, more than 8 billion chickens are slaughtered each year for meat,[78] and more than 300 million chickens are reared for egg production.[79] The vast majority of poultry is raised infactory farms. According to theWorldwatch Institute, 74% of the world's poultry meat and 68% of eggs are produced this way.[80] An alternative to intensive poultry farming isfree-range farming. Friction between these two main methods has led to long-term issues ofethical consumerism. Opponents ofintensive farming argue that it harms the environment, creates human health risks and is inhumane towardssentient animals.[81] 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.[82] Chickens farmed for meat are calledbroilers. Broiler breeds typically take less than six weeks to reach slaughter size,[83] some weeks longer forfree range andorganic broilers.[84]
A commercial chicken house with open sides raising broiler pullets for meat
Chickens farmed primarily for eggs are called layer hens. The UK alone consumes more than 34 million eggs per day.[85] Hens of some breeds can produce over 300 eggs per year; the highest authenticated rate of egg laying is 371 eggs in 364 days.[86] After 12 months of laying, the commercial hen's egg-laying ability declines to the point where the flock is commercially unviable. Hens, particularly frombattery 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.[87] In the UK and Europe, laying hens are then slaughtered and used in processed foods, or sold as 'soup hens'.[87] In some other countries, flocks are sometimesforce moulted rather than being slaughtered to re-invigorate egg-laying. This involves complete withdrawal of food (and sometimes water) for 7–14 days[88] or sufficiently long to cause a body weight loss of 25 to 35%,[89] or up to 28 days under experimental conditions.[90] 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.[91] 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[92] among urban and suburban residents.[93] 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.[94][95] Chickens are social, inquisitive, intelligent[96] birds, and many people find their behaviour entertaining.[97] Certain breeds, such assilkies and manybantam varieties, are generally docile and are often recommended as good pets around children with disabilities.[98]
Acockfight is a contest held in a ring called a cockpit between two cocks. Cockfighting is outlawed in many countries as involvingcruelty to animals.[99] The activity seems to have been practised in theIndus Valley civilisation from 2500 to 2100 BC.[100] In the process of domestication, chickens were apparently kept initially for cockfighting, and only later used for food.[101]
In science
Chickens have long been used asmodel 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 inlimb development have been made using chicken embryos, such as the discovery of theapical ectodermal ridge and thezone of polarizing activity.[102]
The chicken was the first bird species to have itsgenome sequenced.[103] At 1.21Gb, the chicken genome is similarly sized compared to other birds, but smaller than nearly all mammals: thehuman genome is 3.2Gb.[104] The final gene set contained 26,640 genes (including noncoding genes andpseudogenes), with a total of 19,119 protein-coding genes, a similar number to the human genome.[105] In 2006, scientists researching the ancestry of birds switched on a chickenrecessive gene,talpid2, and found that the embryo jaws initiated formation of teeth, like those found in ancient bird fossils.[106]
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