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The chicken (''Gallus gallus domesticus'') is a
domesticated Domestication is a sustained multi-generational relationship in which humans assume a significant degree of control over the reproduction and care of another group of organisms to secure a more predictable supply of resources from that group. A ...
junglefowl Junglefowl are the only four living species of bird from the genus ''Gallus'' in the bird order Galliformes, and occur in parts of South Asia, South and Southeast Asia. They diverged from their common ancestor about 4–6 million years ago. Alt ...
species, with attributes of wild species such as the
grey Grey (more common in British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, " English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to th ...
and the
Ceylon junglefowl The Sri Lankan junglefowl (''Gallus lafayettii'' sometimes spelled ''Gallus lafayetii''), also known as the Ceylon junglefowl or Lafayette's junglefowl, is a member of the Galliformes bird order which is endemic (ecology), endemic to Sri Lanka, w ...
that are originally from
Southeastern Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...
. Rooster or cock is a term for an adult male bird, and a younger male may be called a cockerel. A male that has been
castrated Castration is any action, surgery, surgical, chemical substance, chemical, or otherwise, by which an individual loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy (excision of both testicles), while chemic ...
is a
capon A capon (from la, cāpō, genitive ''cāpōnis'') is a cockerel (rooster) that has been castrated or neutered, either physically or chemically, to improve the quality of its Chicken as food, flesh for food, and, in some countries like Spain, f ...
. An adult female bird is called a hen and a sexually immature female is called a pullet. Humans now keep chickens primarily as a source of food (consuming both their
meat Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chick ...
and
eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...
) and as
pet A pet, or companion animal, is an animal Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce s ...

pet
s. Traditionally they were also bred for
cockfight A cockfight is a blood sport, held in a ring called a cockpit. The history of raising fowl for fighting goes back 6,000 years. The first documented use of the ''word'' gamecock, denoting use of the cock (bird), cock as to a "game", a sport, pas ...

cockfight
ing, which is still practiced in some places. Chickens are one of the most common and widespread
domestic animals This page gives a list of domesticated animals, also including a list of domestication of animals, animals which are or may be currently undergoing the process of domestication and animals that have an extensive relationship with humans beyond simp ...
, with a total population of 23.7 billion , up from more than 19 billion in 2011. There are more chickens in the world than any other
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

bird
. There are numerous
cultural references to chickens There are numerous cultural references to chickens, in myth, folklore and religion, in language and in literature. In Classical Antiquity In Greek mythology, Alectryon (mythology), Alectryon was a young man that Ares put as a guardian outsi ...
– in
myth Myth is a folklore genre consisting of Narrative, narratives that play a fundamental role in a society, such as foundational tales or Origin myth, origin myths. Since "myth" is widely used to imply that a story is not Objectivity (philosophy), ...
,
folklore Folklore is shared by a particular group of people; it encompasses the traditions common to that culture, subculture or group. This includes oral traditions such as Narrative, tales, legends, proverbs and jokes. They include material culture, r ...

folklore
and
religion Religion is usually defined as a social- cultural system of designated behaviors and practices, morals, beliefs, worldviews, texts, sanctified places, prophecies, ethics, or religious organization, organizations, that generally relates hu ...

religion
, and in
language Language is a structured system of communication. The structure of a language is its grammar and the free components are its vocabulary. Languages are the primary means by which humans communicate, and may be conveyed through a variety of met ...

language
and
literature Literature is any collection of Writing, written work, but it is also used more narrowly for writings specifically considered to be an art form, especially prose fiction, drama, and poetry. In recent centuries, the definition has expanded to ...

literature
. Genetic studies have pointed to multiple maternal origin theories of within
South Asia South Asia is the southern subregion of Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental land ...

South Asia
, Southeast Asia, and
East Asia East Asia is the eastern region of Asia, which is defined in both Geography, geographical and culture, ethno-cultural terms. The modern State (polity), states of East Asia include China, Japan, Mongolia, North Korea, South Korea, and Taiwan. ...

East Asia
, but the
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants – on a phylogenetic tree. ...

clade
found in the
Americas The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North America, North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World. ...

Americas
,
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

Europe
, the
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabian Peninsula, Arabia (including the Arabian Peninsula and Bahrain), Anatolia, Asia Minor (Asian part of Turkey except Hatay Pro ...

Middle East
and
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

Africa
originated from the
Indian subcontinent The Indian subcontinent is a list of the physiographic regions of the world, physiographical region in United Nations geoscheme for Asia#Southern Asia, Southern Asia. It is situated on the Indian Plate, projecting southwards into the Indian O ...

Indian subcontinent
. From
ancient India According to consensus in modern genetics, anatomically modern humans first arrived on the Indian subcontinent from Africa between 73,000 and 55,000 years ago. Quote: "Y-Chromosome and Mt-DNA data support the colonization of South Asia by ...

ancient India
, the chicken spread to the Eastern Mediterranean. They appear in
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...

Egypt
in the mid-15th century BC, with the "bird that gives birth every day" having come from the land between
Syria Syria ( ar, سُورِيَا or سُورِيَة, translit=Sūriyā), officially the Syrian Arab Republic ( ar, الجمهورية العربية السورية, al-Jumhūrīyah al-ʻArabīyah as-Sūrīyah), is a Western Asian country loc ...

Syria
and Shinar,
Babylonia Babylonia (; Akkadian: , ''māt Akkadī'') was an ancient Akkadian-speaking state and cultural area based in the city of Babylon in central-southern Mesopotamia (present-day Iraq and parts of Syria). It emerged as an Amorites, Amorite-ruled ...
, according to the
annals of Thutmose III The Annals of Thutmose III are composed of numerous inscription Epigraphy () is the study of inscriptions, or epigraphs, as writing; it is the science of identifying graphemes, clarifying their meanings, classifying their uses according to d ...
. They are known in
Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...

Greece
from the 5th century BC. In 2011 a study in genetic and archaeological evidence conclude that the origin of the modern-day chicken is from
Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical United Nations geoscheme for Asia#South-eastern Asia, south-eastern region of Asia, consistin ...

Southeast Asia
.


Terminology

An adult male is a called a ''cock'' or (in the United States) a ''rooster'' and an adult female is called a ''hen''. Other terms are: * ''Biddy'': a newly hatched chicken * ''Capon'': a castrated or
neutered Neutering, from the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around p ...
male chicken * '' Chick'': a young chicken * ''Chook'' : a chicken (Australia/New Zealand, informal) * ''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. * ''Yardbird'': a chicken (southern United States, dialectal) ''Chicken'' may also mean a ''chick'' . In fact, ''chicken'' was originally a term only for an immature, or at least young, bird. In older sources, chicken as a species were typically referred to as ''common fowl'' or ''domestic fowl''. In Australian or New Zealand vernacular English the wor
''chook''
provides the generic term for the species (e.g. "a cooked chook" or "she keeps chooks"); which enables ''chicken'' to commonly 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


Etymology

According to Merriam-Webster, the term ''rooster'' (i.e. a roosting bird) originated in the mid- or late 18th century as a euphemism to avoid the sexual connotation of the original English '' cock'',Hugh Rawson
"Why Do We Say...? Rooster", ''American Heritage'', August–September 2006.
Online Etymology Dictionary
Entry for ''rooster (n.)'', May 2019
and is widely used throughout North America. ''Roosting'' is the action of perching aloft to sleep at night.


General biology and habitat

Chickens are
omnivore An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and Dietary fiber, fiber, and metab ...
s. In the wild, they often scratch at the soil to search for seeds, insects, and even animals as large as
lizard Lizards are a widespread group of Squamata, squamate reptiles, with over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The group is paraphyletic since it excludes the snakes and Amphisbae ...

lizard
s, small snakes, or sometimes young
mice A mouse (plural, : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus mus ...

mice
. The average chicken may live for 5–10 years, depending on the
breed A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slight ...
. The world's oldest known chicken lived 16 years according to
Guinness World Records ''Guinness World Records'', known from its inception in 1955 until 1999 as ''The Guinness Book of Records'' and in previous United States editions as ''The Guinness Book of World Records'', is a reference book published annually, listing world ...
. Roosters can usually be differentiated from hens by their striking plumage of long flowing tails and shiny, pointed feathers on their necks ('hackles') and backs ('saddle'), which are typically of brighter, bolder colours than those of females of the same breed. However, in some breeds, such as the
Sebright chicken
Sebright chicken
, the rooster has only slightly pointed neck feathers, the same colour as the hen's. The identification can be made by looking at the
comb A comb is a tool consisting of a shaft that holds a row of teeth for pulling through the hair to clean, untangle, or style it. Combs have been used since Prehistory, prehistoric times, having been discovered in very refined forms from settlemen ...
, or eventually from the development of spurs on the male's legs (in a few breeds and in certain hybrids, the male and female chicks may be differentiated by colour). Adult chickens have a fleshy crest on their heads called a comb, or cockscomb, and hanging flaps of skin 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 In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
found in several chicken breeds which causes extra
feather Feathers are epidermis (zoology), epidermal growths that form a distinctive outer covering, or plumage, on both Bird, avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in ...

feather
ing under the chicken's face, giving the appearance of a
beard A beard is the hair that grows on the jaw, chin, upper lip, lower lip, cheeks, and neck of humans and some non-human animals. In humans, usually pubescent or adult males are able to grow beards. Throughout the course of history, societal at ...

beard
. Domestic chickens are not capable of long-distance flight, although lighter chickens are generally capable of flying for short distances, such as over fences or into trees (where they would naturally roost). Chickens may occasionally fly briefly to explore their surroundings, but generally do so only to flee perceived danger.


Behavior


Social behaviour

Chickens are
gregarious Sociality is the degree to which individuals in an animal population tend to associate in social groups (gregariousness) and form cooperative societies. Sociality is a survival response to evolutionary pressures. For example, when a mother wasp ...
birds and live together in
flocks
flocks
. They have a communal approach to the
incubation The word incubation may refer to: Science and technology * Egg incubation, sitting on or brooding the eggs of birds and other egg-laying animals to hatch them * Incubation (psychology), the process of thinking about a problem subconsciously whi ...
of eggs and raising of young. Individual chickens in a flock will dominate others, establishing a '
pecking order In biology, a dominance hierarchy (formerly and colloquially called a pecking order) is a type of social hierarchy that arises when members of animal social animal , social groups interact, creating a ranking system. A dominant higher-ranking i ...
', with dominant individuals having priority for food access and nesting locations. Removing hens or roosters from a flock causes a temporary disruption to this social order until a new pecking order is established. Adding hens, especially younger birds, to an existing flock can lead to fighting and injury.


Vocalizations

When a rooster finds food, he may call other chickens to eat first. He does this by clucking in a high pitch as well as picking up and dropping the food. This behaviour may also be observed in mother hens to call their chicks and encourage them to eat. A rooster's crowing is a loud and sometimes shrill call and sends 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 also to call their chicks. Chickens also give different warning calls when they sense a predator approaching from the air or on the ground.


= Crowing

= Roosters almost always start crowing before four months of age. Although it is possible for a hen to crow as well, crowing (together with hackles development) is one of the clearest signs of being a rooster.


Rooster crowing contests

Rooster crowing contests, also known as crowing contests, are a traditional sport in several countries, such as Germany, the Netherlands, Belgium, the United States,
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. It consists of over List of islands of Indonesia, 17,000 islands, including Sumatr ...

Indonesia
and Japan. The oldest contests are held with longcrowers. Depending on the breed, either the duration of the crowing or the times the rooster crows within a certain time is measured.


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: # Male approaching the hen # Male pre-copulatory waltzing # Male waltzing # Female crouching (receptive posture) or stepping aside or running away (if unwilling to copulate) # Male mounting # Male treading with both feet on hen's back # Male tail bending (following successful copulation)


Nesting and laying behaviour

Hens will often try to lay in nests that already contain eggs and have been known to move eggs from neighbouring nests into their own. The result of this behaviour is that a flock will use only a few preferred locations, rather than having a different nest for every bird. Hens will often express a preference to lay in the same location. It is not unknown for two (or more) hens to try to share the same nest at the same time. If the nest is small, or one of the hens is particularly determined, this may result in chickens trying 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 A clutch is a mechanical device that engages and disengages power Transmission (mechanics), transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (driv ...
is complete, and they will then incubate all the eggs. Hens are then said to "go broody". The broody hen will stop laying and instead will focus on the incubation of the eggs (a full clutch is usually about 12 eggs). She will sit or 'set' on the nest, fluffing up or pecking in defense if disturbed or removed. The hen will rarely leave the nest to eat, drink, or dust-bathe. While brooding, the hen maintains the nest at a constant temperature and humidity, as well as turning the eggs regularly during the first part of the incubation. To stimulate broodiness, owners may place several artificial eggs in the nest. To discourage it, they may place the hen in an elevated cage with an open wire floor.
Breed A breed is a specific group of domestic animals having homogeneous appearance (phenotype), homogeneous behavior, and/or other characteristics that distinguish it from other organisms of the same species. In literature, there exist several slight ...
s artificially developed for egg production rarely go broody, and those that do often stop part-way through the incubation. However, other breeds, such as the
Cochin Kochi (), also known as Cochin ( ) (List of renamed Indian cities and states#Kerala, the official name until 1996) is a major port city on the Malabar Coast of India bordering the Laccadive Sea, which is a part of the Arabian Sea. It is part ...
, Cornish and
Silkie The Silkie (also known as the Silky or Chinese silk chicken) is a List of chicken breeds, breed of chicken named for its atypically fluffy plumage, which is said to feel like silk and satin. The breed has several other unusual qualities, such ...
, do regularly go broody, and make excellent mothers, not only for chicken eggs but also for those of other species — even those with much smaller or larger eggs and different incubation periods, such as
quail Quail is a collective name for several genera of mid-sized birds generally placed in the order Galliformes. The collective noun for a group of quail is a flock (birds), flock, covey, or bevy. Old World quail are placed in the family Phasianida ...

quail
,
pheasant Pheasants ( ) are birds of several genera within the family (biology), family Phasianidae in the order (biology), order Galliformes. Although they can be found all over the world in introduced (and captive) populations, the pheasant genera na ...

pheasant
s,
ducks Duck is the common name for numerous species of waterfowl in the family (biology), family Anatidae. Ducks are generally smaller and shorter-necked than swans and goose, geese, which are members of the same family. Divided among several subfam ...

ducks
,
turkeys The turkey is a large bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a hig ...
, or
geese A goose (plural, : geese) is a bird of any of several waterfowl species in the family (biology), family Anatidae. This group comprises the genera ''Anser (bird), Anser'' (the grey geese and white geese) and ''Branta'' (the black geese). Some o ...

geese
.


Hatching and early life

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 An egg tooth is a temporary, sharp projection present on the beak, bill or snout of an oviparity, oviparous animal at hatching. It allows the hatchling to penetrate the eggshell from inside and break free. Birds, reptiles, and monotremes possess ...
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 The yolk sac is a membranous sac attached to an embryo An embryo is an initial stage of development of a multicellular organism. In organisms that Sexual reproduction, reproduce sexually, embryonic development is the part of the life cycle ...
. 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.


Defensive behaviour

Chickens may occasionally gang up on a weak or inexperienced predator. At least one credible report exists of a young fox killed by hens. A group of hens have been recorded in attacking a
hawk Hawks are bird of prey, birds of prey of the family Accipitridae. They are widely distributed and are found on all continents except Antarctica. * The subfamily Accipitrinae includes goshawks, sparrowhawks, sharp-shinned hawks and others. Th ...

hawk
that had entered their coop. If a chicken is threatened by predators, stress, or is sick, there is a chance that they will puff up their feathers.


Reproduction

Sperm transfer occurs by
cloaca In animal anatomy, a cloaca ( ), plural cloacae ( or ), is the posterior orifice that serves as the only opening for the digestive, reproductive, and urinary tracts (if present) of many vertebrate animals. All amphibians, reptiles and bir ...
l 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 Neuroendocrine cells are cells that receive neuronal input (through neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, any ...
system, the Gonadotropin-Releasing Hormone-I neurons in the
hypothalamus The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalam ...

hypothalamus
. Locally to the
reproductive system The reproductive system of an organism, also known as the genital system, is the biological system made up of all the anatomical sex organs, organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pherom ...
itself, reproductive hormones such as
estrogen Estrogen or oestrogen is a category of sex steroid, sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogeny (biology), endogenous estrogens th ...

estrogen
,
progesterone Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the major ...

progesterone
,
gonadotropin Gonadotropins are glycoprotein hormones secreted by gonadotropic cells of the anterior pituitary of vertebrates. This family includes the mammalian hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the placental/chorionic ...
s (
luteinizing hormone Luteinizing hormone (LH, also known as luteinising hormone, lutropin and sometimes lutrophin) is a hormone produced by gonadotropic cells in the anterior pituitary gland. The production of LH is regulated by gonadotropin-releasing hormone (GnRH) ...
and
follicle-stimulating hormone Follicle-stimulating hormone (FSH) is a gonadotropin, a glycoprotein polypeptide hormone. FSH is synthesized and secreted by the gonadotropic cells of the anterior pituitary gland and regulates the development, growth, pubertal maturation ...
) 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

Chicken embryos have long been used as
model organism A model organism (often shortened to model) is a non-human species that is extensively studied to understand particular biology, biological phenomena, with the expectation that discoveries made in the model organism will provide insight into th ...
s 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 Limb development in vertebrates is an area of active research in both developmental biology, developmental and evolutionary biology, with much of the latter work focused on the transition from fin to limb (anatomy), limb. Limb formation begins in ...
have been made using chicken embryos, such as the discovery of the
apical ectodermal ridge The apical ectodermal ridge (AER) is a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb. After the limb bud induces AER formation, the AE ...
(AER) and the
zone of polarizing activity The zone of polarizing activity (ZPA) is an area of mesenchyme that contains signals which instruct the developing limb bud to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an ectoderm covering. Event ...
(ZPA) by John W. Saunders. In 2006, scientists researching the ancestry of birds "turned on" a chicken
recessive gene In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the Phenotype, effect of a different variant of the same gene on Homologous chromosome, the other copy of the chromosome. The first ...
, ''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

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 considerably smaller than other vertebrate genomes, such as the
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual Mitochondrial DNA, mitochondria. These are usually treated s ...

human genome
(3 Gb). The final gene set contained 26,640 genes (including noncoding genes and
pseudogene Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by Reverse transcriptase, reverse transcription of an mRNA trans ...
s), 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 Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...

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 The pineal gland, conarium, or epiphysis cerebri, is a small endocrine gland in the brain of most vertebrates. The pineal gland produces melatonin, a serotonin-derived hormone which modulates sleep, sleep patterns in both circadian rhythm, circ ...

pineal gland
.


Immunology

Although all appear to have lost
TLR9 Toll-like receptor 9 is a protein that in humans is encoded by the ''TLR9'' gene. TLR9 has also been designated as CD289 (cluster of differentiation 289). It is a member of the toll-like receptor (TLR) family. TLR9 is an important receptor expresse ...
, artificial immunity against bacterial pathogens has been induced in neonatal chicks by Taghavi et al. 2008 using tailored oligodeoxynucleotides.


Origin and dispersal


Origin

Galliformes Galliformes is an order of heavy-bodied ground-feeding bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Egg ...

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
partridge A partridge is a medium-sized Galliformes, galliform bird in any of several genera, with a wide Indigenous (ecology), native distribution throughout parts of Europe, Asia and Africa. Several species have been introduced to the Americas. They a ...

partridge
s, survived the
Cretaceous–Paleogene extinction event The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden extinction event, mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million y ...
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 The red junglefowl (''Gallus gallus'') is a tropical bird in the family Phasianidae. It ranges across much of Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-e ...

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 The gray junglefowl (''Gallus sonneratii''), also known as Sonnerat's junglefowl, is one of the wild ancestors of the Chicken, domestic chicken together with the red junglefowl and other junglefowls. The species epithet commemorates the French e ...
,
Sri Lankan junglefowl The Sri Lankan junglefowl (''Gallus lafayettii'' sometimes spelled ''Gallus lafayetii''), also known as the Ceylon junglefowl or Lafayette's junglefowl, is a member of the Galliformes Galliformes is an order of heavy-bodied ground-feeding b ...

Sri Lankan junglefowl
and
green junglefowl The green junglefowl (''Gallus varius''), also known as Javan junglefowl, forktail or green Javanese junglefowl, is the most distantly related and the first to diverge at least 4 million years ago among the four species of the 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% - 79% of their genome with red junglefowl, with the period of domestication dated to 8,000 years ago.


Domestication

In the last decade, there have been a number of genetic studies to clarify the origins. According to one early study, a single domestication event of the red junglefowl in what now is the country of
Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Mainland Southeast Asia, Indochinese Peninsula, spanning , with a population of almost 70 mi ...

Thailand
gave rise to the modern chicken with minor transitions separating the modern breeds. The red junglefowl, known as the bamboo fowl in many Southeast Asian languages, 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 a controversial issue. Genomic studies estimate that the chicken was domesticated 8,000 years ago in Southeast Asia and spread to China and India 2000–3000 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

Middle Eastern chicken remains go back to a little earlier than 2000 BC in
Syria Syria ( ar, سُورِيَا or سُورِيَة, translit=Sūriyā), officially the Syrian Arab Republic ( ar, الجمهورية العربية السورية, al-Jumhūrīyah al-ʻArabīyah as-Sūrīyah), is a Western Asian country loc ...

Syria
; chickens went southward only in the 1st millennium BC. They reached
Egypt Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a List of transcontinental countries, transcontinental country spanning the North Africa, northeast corner of Africa and Western Asia, southwest corner of Asia via a land bridg ...

Egypt
for purposes of
cockfighting A cockfight is a blood sport, held in a ring called a cockpit. The history of raising fowl for fighting goes back 6,000 years. The first documented use of the ''word'' gamecock, denoting use of the cock (bird), cock as to a "game", a sport, pas ...

cockfighting
about 1400 BC, and became widely bred only in Ptolemaic Egypt (about 300 BC).CHOF : The Cambridge History of Food, 2000,
Cambridge University Press Cambridge University Press is the university press of the University of Cambridge. Granted letters patent by Henry VIII of England, King Henry VIII in 1534, it is the oldest university press in the world. It is also the King's Printer. Cambr ...
, vol.1, pp496-499
Phoenicians spread chickens along the Mediterranean coasts as far as Iberia. During the
Hellenistic period In Classical antiquity, the Hellenistic period covers the time in History of the Mediterranean region, Mediterranean history after Classical Greece, between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as sig ...
(4th–2nd centuries BC), in the Southern Levant, chickens began to be widely domesticated for food. This change occurred at least 100 years before domestication of chickens spread to Europe. Chickens reached Europe circa 100 BC. Breeding increased under the
Roman Empire The Roman Empire ( la, Imperium Romanum ; grc-gre, Βασιλεία τῶν Ῥωμαίων, Basileía tôn Rhōmaíōn) was the post-Roman Republic, Republican period of ancient Rome. As a polity, it included large territorial holdings aro ...

Roman Empire
, and was reduced in the
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the Post-classical, post-classical period of World history (field), global history. It began with t ...
. DNA sequencing, 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 River, Nile Valley, the East Africa Roman-Greek or Indian trade, or from Carthage and the Berbers, across the Sahara Desert, 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. Domestic chicken in the Americas before Western contact is still an ongoing discussion, 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.


South America

An unusual variety of chicken that has its origins in South America is the Araucana, bred in southern Chile by the Mapuche people. Araucanas lay blue-green eggs. Additionally, some Araucanas are tailless, and some have tufts of feathers around their ears. It has long been suggested that they pre-date the arrival of European chickens brought by the Spanish colonization of the Americas, Spanish and are evidence of pre-Columbian trans-Pacific contacts between Asian or Pacific Oceanic peoples, particularly the Polynesians, and South America. In 2007, an international team of researchers reported the results of their analysis of chicken bones found on the Arauco Peninsula in Zona Sur, south-central Chile. Radiocarbon dating suggested that the chickens were pre-Columbian, and DNA analysis showed that they were related to prehistoric populations of chickens in Polynesia. These results appeared to confirm that the chickens came from Polynesia and that there were transpacific contacts between Polynesia and South America before Columbus's arrival in the Americas. However, a later report looking at the same specimens concluded:
A published, apparently pre-Columbian, Chilean specimen and six pre-European Polynesian specimens also cluster with the same European/Indian subcontinental/Southeast Asian sequences, providing no support for a Polynesian introduction of chickens to South America. In contrast, sequences from two archaeological sites on Easter Island group with an uncommon haplogroup from Indonesia, Japan, and China and may represent a genetic signature of an early Polynesian dispersal. Modeling of the potential marine carbon contribution to the Chilean archaeological specimen casts further doubt on claims for pre-Columbian chickens, and definitive proof will require further analyses of ancient DNA sequences and radiocarbon and stable isotope data from archaeological excavations within both Chile and Polynesia.
The debate for and against a Polynesian origin for South American chickens continued with this 2014 paper and subsequent responses in ''PNAS''.


Use by humans


Farming

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 are raised in Intensive animal farming, factory farms. According to the Worldwatch Institute, 74 percent of the world's poultry meat and 68 percent 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

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 (food), organic broiler will usually be slaughtered at about 14 weeks of age.


Reared for eggs

Chickens farmed primarily for eggs are called layer hens. In total, the UK alone consumes more than 34 million eggs per day. Some hen List of chicken breeds, 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 Forced molting, 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 wikt:urban, 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 find their behaviour entertaining. Certain breeds, such as Silkies and many Bantam (poultry), 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

A cockfight is a contest held in a ring called a cockpit between two cocks known as ''gamecocks.'' This term, denoting a cock kept for game, sport, pastime or entertainment, appears in 1646, after "cock of the game" used by George Wilson in the earliest known book on the secular sport, ''The Commendation of Cocks and Cock Fighting'' of 1607. Gamecocks are not typical farm chickens. The cocks are specially bred and trained for increased stamina and strength. The comb and Wattle (anatomy), wattle are removed from a young gamecock because, if left intact, they would be a disadvantage during a match. This process is called Dubbing (poultry), dubbing. Sometimes the cocks are given drugs to increase their stamina or thicken their blood, which increases their chances of winning. Cockfighting is considered a traditional sporting event by some, and an example of animal cruelty by others and is therefore outlawed in most countries. Usually wagers are made on the outcome of the match, with the survivor or last bird standing declared winner. Chickens were originally used for cockfighting, a sport where 2 male chickens (cocks) fight each other until one dies or becomes badly injured. Cocks possess congenital aggression toward all other cocks to contest with females. Studies suggest that cockfights have existed even up to the Indus Valley civilisation as a pastime.Sherman, David M. (2002). ''Tending Animals in the Global Village''. Blackwell Publishing. 46. . Today it is commonly associated with religious worship, pastime, and gambling in Asian and some South American countries. While not all fights are to the death, most use metal spurs as a weapon attached above or below the chicken's own spur, which typically results in death in one or both cocks. If chickens are in practice, owners place gloves on the spurs to prevent injuries. Cockfighting has been banned in most western countries and debated by animal rights activists for its brutality.


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 from the optimum temperature of will reduce hatch rates. Humidity is also 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. The position of the eggs in the incubator can also influence hatch rates. For best results, eggs should be placed with the pointed ends down and turned regularly (at least three times per day) until one to three days before hatching. If the eggs aren't turned, the embryo inside may stick to the shell and may hatch with physical defects. Adequate ventilation is necessary to provide the embryo with oxygen. Older eggs require increased ventilation. Many commercial incubators are industrial-sized with shelves holding tens of thousands of eggs at a time, with rotation of the eggs a fully automated process. Home incubators are boxes holding from 6 to 75 eggs; they are usually electrically powered, but in the past some were heated with an oil or paraffin lamp.


Diseases and ailments

Chickens are susceptible to several parasites, including lice, mites, ticks, fleas, and Roundworm, intestinal worms, as well as other diseases. Despite the name, they are not affected by chickenpox, which is generally restricted to humans. Chickens can carry and transmit salmonella in their dander and feces. In the United States, the Centers for Disease Control and Prevention advise against bringing them indoors or letting small children handle them. Some of the diseases that can affect chickens are shown below:


History

An early domestication of chickens in Southeast Asia is probable, since the word for domestic chicken (''*manuk'') is part of the reconstructed Proto-Austronesian language . Chickens, together with dogs and pigs, were the domestic animals of the Lapita culture, the first Neolithic culture of Oceania. The first pictures of chickens in Europe are found on Corinth, Greece, Corinthian pottery of the 7th century BC. Chickens were spread by Polynesian seafarers and reached Easter Island in the 12th century AD, where they were the only domestic animal, with the possible exception of the Polynesian rat (''Rattus exulans''). They were housed in extremely solid chicken coops built from stone, which was first reported as such to Linton Palmer in 1868, who also "expressed his doubts about this".


Gallery

File:Brown Leghorn rooster in Australia.jpg, Leghorn chicken, Brown Leghorn in Australia File:Joseph Crawhall - Spanish Cock And Snail.jpg, Joseph Crawhall III, ''Spanish Cock and Snail'' File:Lavender Orpington Chicken.jpg, Two Orpington chicken, Lavender Orpington chicks in Ontario, Canada. File:More chicks.jpg, A group of chicks


Explanatory notes


References

{{Authority control Chickens, Birds described in 1758 Bird common names Taxa named by Carl Linnaeus Articles containing video clips Junglefowls Poultry Subspecies Cosmopolitan birds National symbols of Kenya Heraldic beasts