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 and Southeast Asia. They diverged from their common ancestor about 4–6 million years ago.
Although origin ...
species, with attributes of wild species such as the
grey
Grey (more common in British English) or gray (more common in American English) is an intermediate color between black and white. It is a neutral or achromatic color, meaning literally that it is "without color", because it can be composed o ...
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 to Sri Lanka, where it is the nati ...
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 south-eastern region of Asia, consisting of the regions that are situated south of mainland ...
. 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, surgical, 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 chemical castration uses pharmac ...
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 flesh for food, and, in some countries like Spain, fattened by for ...
. 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 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 kept primarily for a person's company or entertainment rather than as a working animal, livestock, or a laboratory animal. Popular pets are often considered to have attractive appearances, intelligence ...
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 as to a "game", a sport, pastime or ent ...
ing, which is still practiced in some places.
Chickens are one of the most common and widespread
domestic animals, 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 Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
. There are numerous
cultural references to chickens – in
myth,
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 tales, legends, proverbs and jokes. They include material culture, ranging ...
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 organizations, that generally relates humanity to supernatural, ...
, 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 ...
and
literature
Literature is any collection of 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 include ...
.
Genetic studies have pointed to multiple maternal origin theories of within
South Asia
South Asia is the southern subregion of Asia, which is defined in both geographical
Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth descr ...
, 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. ...
,
but the
clade found in the
Americas
The Americas, which are sometimes collectively called America, are a landmass comprising the totality of North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World.
Along with th ...
,
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 subcontinent of Eurasia and it is located entirel ...
, the
Middle East
The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabia (including the Arabian Peninsula and Bahrain), Asia Minor (Asian part of Turkey except Hatay Province), East Thrace (Europ ...
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 ...
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 ...
. 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 m ...
, the chicken spread to the Eastern Mediterranean. They appear in
Egypt
Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
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
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 ...
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 south-eastern region of Asia, consisting of the regions that are situated south of mainlan ...
.
Terminology
![Chicken anatomy](https://upload.wikimedia.org/wikipedia/commons/0/08/Chicken_anatomy.jpg)
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 ''neuter'' ('of neither sex'), is the removal of an animal's reproductive organ, either all of it or a considerably large part. The male-specific term is castration, while spaying is usually reserved for female animals. C ...
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](_blank)
"Why Do We Say...? Rooster", ''American Heritage'', August–September 2006.[Online Etymology Dictionary](_blank)
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
omnivores. In the wild, they often scratch at the soil to search for seeds, insects, and even animals as large as
lizards, small snakes, or sometimes young
mice.
The average chicken may live for 5–10 years, depending on the
breed. 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 ...
.
![Didactic model of a chicken--FMVZ USP-29](https://upload.wikimedia.org/wikipedia/commons/4/45/Didactic_model_of_a_chicken--FMVZ_USP-29.jpeg)
![Eier verschiedener Hühnerrassen](https://upload.wikimedia.org/wikipedia/commons/f/fe/Eier_verschiedener_H%C3%BChnerrassen.jpg)
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
The Sebright (IPA: ) is a British breed of bantam chicken. It is a true bantam – a miniature bird with no corresponding large version – and is one of the oldest recorded British bantam breeds. It is named after Sir John Saunders Sebrigh ...
, 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 prehistoric times, having been discovered in very refined forms from settlements dating ba ...
, 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
feathering 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 ...
.
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
![Cascais Costa do Esteril 52 (36583204550)](https://upload.wikimedia.org/wikipedia/commons/b/b8/Cascais_Costa_do_Esteril_52_%2836583204550%29.jpg)
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
Flocking is the behaviour exhibited when a group of birds, called a flock, are foraging or in flight.
Computer simulations and mathematical models that have been developed to emulate the flocking behaviours of birds can also generally be applie ...
. They have a communal approach to the
incubation 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 groups interact, creating a ranking system. A dominant higher-ranking individual is so ...
', 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
=
![Canterino_Gallo](https://upload.wikimedia.org/wikipedia/commons/d/d1/Canterino_Gallo.jpg)
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 and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
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
![Chicken eggs](https://upload.wikimedia.org/wikipedia/commons/0/0e/Chicken_eggs.jpg)
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 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.
Breeds artificially developed for
egg production
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 ...
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 ( ) ( 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 of the district of Ernakulam in the state of K ...
,
Cornish and
Silkie
The Silkie (also known as the Silky or Chinese silk chicken) is a 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 as black skin and bones, ...
, 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, covey, or bevy.
Old World quail are placed in the family Phasianidae, and New ...
,
pheasant
Pheasants ( ) are birds of several genera within the family Phasianidae in the order Galliformes. Although they can be found all over the world in introduced (and captive) populations, the pheasant genera native range is restricted to Eurasia ...
s,
ducks,
turkeys
The turkey is a large bird in the genus ''Meleagris'', native to North America. There are two extant turkey species: the wild turkey (''Meleagris gallopavo'') of eastern and central North America and the ocellated turkey (''Meleagris ocellat ...
, or
geese
A goose ( : geese) is a bird of any of several waterfowl species in the family Anatidae. This group comprises the genera '' Anser'' (the grey geese and white geese) and ''Branta'' (the black geese). Some other birds, mostly related to the she ...
.
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 bill or snout of an oviparous animal at hatching. It allows the hatchling to penetrate the eggshell from inside and break free. Birds, reptiles, and monotremes possess egg teeth as ha ...
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, formed by cells of the hypoblast layer of the bilaminar embryonic disc. This is alternatively called the umbilical vesicle by the Terminologia Embryologica (TE), though ''yolk sac'' is ...
. 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 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
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
Neuroendocrine cells are cells that receive neuronal input (through neurotransmitters released by nerve cells or neurosecretory cells) and, as a consequence of this input, release messenger molecules (hormones) into the blood. In this way they bri ...
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 hypothalamu ...
. 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 organs involved in sexual reproduction. Many non-living substances such as fluids, hormones, and pheromones are als ...
itself, reproductive hormones such as
estrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...
,
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/ chorioni ...
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) 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 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
Limb development in vertebrates is an area of active research in both developmental and evolutionary biology, with much of the latter work focused on the transition from fin to limb.
Limb formation begins in the morphogenetic limb field, as mes ...
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. Ev ...
(ZPA) 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
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 mitochondria. These are usually treated separately as the ...
(3 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
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 (erythrocyt ...
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.
Immunology
Although all
avians
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
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 birds that includes turkeys, chickens, quail, and other landfowl. Gallinaceous birds, as they are called, are important in their ecosystems as seed dispersers and predators, and are ofte ...
, 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 galliform bird in any of several genera, with a wide native distribution throughout parts of Europe, Asia and Africa. Several species have been introduced to the Americas. They are sometimes grouped in the Perd ...
s, survived the
Cretaceous–Paleogene extinction event
The Cretaceous–Paleogene (K–Pg) extinction event (also known as the Cretaceous–Tertiary extinction) was a sudden mass extinction of three-quarters of the plant and animal species on Earth, approximately 66 million years ago. With the ...
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 and parts of South Asia. It was formerly known as the Bankiva or Bankiva Fowl. It is the species that gave rise to the ...
(''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
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 Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...
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; chickens went southward only in the 1st millennium BC. They reached
Egypt
Egypt ( ar, مصر , ), officially the Arab Republic of Egypt, is a transcontinental country spanning the northeast corner of Africa and southwest corner of Asia via a land bridge formed by the Sinai Peninsula. It is bordered by the Medit ...
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 as to a "game", a sport, pastime or ente ...
about 1400 BC, and became widely bred only in
Ptolemaic Ptolemaic is the adjective formed from the name Ptolemy, and may refer to:
Pertaining to the Ptolemaic dynasty
* Ptolemaic dynasty, the Macedonian Greek dynasty that ruled Egypt founded in 305 BC by Ptolemy I Soter
* Ptolemaic Kingdom
Pertaining ...
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 King Henry VIII in 1534, it is the oldest university press in the world. It is also the King's Printer.
Cambridge University Pre ...
, 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 Mediterranean history after Classical Greece, between the death of Alexander the Great in 323 BC and the emergence of the Roman Empire, as signified by the Battle of Actium in ...
(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- Republican period of ancient Rome. As a polity, it included large territorial holdings around the Mediter ...
, 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 period of global history. It began with the fall of the Western Roman Empire ...
.
Genetic sequencing of chicken bones from archaeological sites in Europe revealed that in the
High Middle Ages
The High Middle Ages, or High Medieval Period, was the period of European history that lasted from AD 1000 to 1300. The High Middle Ages were preceded by the Early Middle Ages and were followed by the Late Middle Ages, which ended around AD 150 ...
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
The Nile, , Bohairic , lg, Kiira , Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered the longest ...
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.
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
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
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
The Araucana ( es, Gallina Mapuche, italic=no) is a breed of domestic chicken from Chile. Its name derives from the Araucanía region of Chile where it is believed to have originated. It lays blue-shelled eggs, one of very few breeds that do so. ...
, bred in southern
Chile
Chile, officially the Republic of Chile, is a country in the western part of South America. It is the southernmost country in the world, and the closest to Antarctica, occupying a long and narrow strip of land between the Andes to the east a ...
by the
Mapuche
The Mapuche ( (Mapuche & Spanish: )) are a group of indigenous inhabitants of south-central Chile and southwestern Argentina, including parts of Patagonia. The collective term refers to a wide-ranging ethnicity composed of various groups who s ...
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
Spanish might refer to:
* Items from or related to Spain:
**Spaniards are a nation and ethnic group indigenous to Spain
**Spanish language, spoken in Spain and many Latin American countries
**Spanish cuisine
Other places
* Spanish, Ontario, Can ...
and are evidence of
pre-Columbian
In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic period through European colonization, which began with Christopher Columbus's voyage of 1492. Usually, ...
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 The Arauco Peninsula (Spanish: península de Arauco), is a peninsula in Southern Chile located in the homonymous Arauco Province. It projects northwest into the Pacific Ocean. The peninsula is located west of Cordillera de Nahuelbuta. Geologicall ...
in
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
![BatteryChicken5DaysOutOfCage](https://upload.wikimedia.org/wikipedia/commons/8/8f/BatteryChicken5DaysOutOfCage.jpg)
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
factory farms. According to the
Worldwatch Institute
The Worldwatch Institute was a globally focused environmental research organization based in Washington, D.C., founded by Lester R. Brown. Worldwatch was named as one of the top ten sustainable development research organizations by Globescan Su ...
, 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
Free range denotes a method of farming husbandry where the animals, for at least part of the day, can roam freely outdoors, rather than being confined in an enclosure for 24 hours each day.
On many farms, the outdoors ranging area is fenced, ...
farming.
Friction between these two main methods has led to long-term issues of
ethical consumerism
Ethical consumerism (alternatively called ethical consumption, ethical purchasing, moral purchasing, ethical sourcing, or ethical shopping and also associated with sustainable and green consumerism) is a type of consumer activism based on the con ...
. Opponents of
intensive farming
Intensive agriculture, also known as intensive farming (as opposed to extensive farming), conventional, or industrial agriculture, is a type of agriculture, both of crop plants and of animals, with higher levels of input and output per unit of ...
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
![Florida chicken house](https://upload.wikimedia.org/wikipedia/commons/7/78/Florida_chicken_house.jpg)
Chickens farmed for meat are called
broiler
A broiler is any chicken (''Gallus gallus domesticus'') that is bred and raised specifically for meat production. Most commercial broilers reach slaughter weight between four and six weeks of age, although slower growing breeds reach slaught ...
s. 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
Free range denotes a method of farming husbandry where the animals, for at least part of the day, can roam freely outdoors, rather than being confined in an enclosure for 24 hours each day.
On many farms, the outdoors ranging area is fenced, ...
or
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
breeds
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 ...
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
Battery cages are a housing system used for various animal production methods, but primarily for egg-laying hens. The name arises from the arrangement of rows and columns of identical cages connected together, in a unit, as in an artillery batt ...
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
![A 95 year old woman with her pet rooster, Havana, Cuba](https://upload.wikimedia.org/wikipedia/commons/9/92/A_95_year_old_woman_with_her_pet_rooster%2C_Havana%2C_Cuba.jpg)
Keeping chickens as pets became increasingly popular in the 2000s among
urban
Urban means "related to a city". In that sense, the term may refer to:
* Urban area, geographical area distinct from rural areas
* Urban culture, the culture of towns and cities
Urban may also refer to:
General
* Urban (name), a list of people ...
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
Silkie
The Silkie (also known as the Silky or Chinese silk chicken) is a 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 as black skin and bones, ...
s 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
![Cocks_Fighting](https://upload.wikimedia.org/wikipedia/commons/e/ef/Cocks_Fighting.jpg)
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 are removed from a young gamecock because, if left intact, they would be a disadvantage during a match. This process is called
dubbing
Dubbing (re-recording and mixing) is a post-production process used in filmmaking and video production, often in concert with sound design, in which additional or supplementary recordings are lip-synced and "mixed" with original production sou ...
. 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
Cruelty to animals, also called animal abuse, animal neglect or animal cruelty, is the infliction by omission (neglect) or by commission by humans of suffering or harm upon non-human animals. More narrowly, it can be the causing of harm or su ...
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
The Indus Valley Civilisation (IVC), also known as the Indus Civilisation was a Bronze Age civilisation in the northwestern regions of South Asia, lasting from 3300 BCE to 1300 BCE, and in its mature form 2600 BCE to 1900& ...
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
![Egg incubator](https://upload.wikimedia.org/wikipedia/commons/5/53/Egg_incubator.jpg)
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
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present.
Humidity dep ...
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
parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
s, including
lice,
mite
Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear e ...
s,
tick
Ticks (order Ixodida) are parasitic arachnids that are part of the mite superorder Parasitiformes. Adult ticks are approximately 3 to 5 mm in length depending on age, sex, species, and "fullness". Ticks are external parasites, living by ...
s,
flea
Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about long, a ...
s, and
intestinal worms
An intestinal parasite infection is a condition in which a parasite infects the gastro-intestinal tract of humans and other animals. Such parasites can live anywhere in the body, but most prefer the intestinal wall.
Routes of exposure and infe ...
, 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
The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...
advise against bringing them indoors or letting small children handle them.
Some of the diseases that can affect chickens are shown below:
History
![Red junglefowl hm](https://upload.wikimedia.org/wikipedia/commons/7/75/Red_junglefowl_hm.jpg)
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
Proto-Austronesian (commonly abbreviated as PAN or PAn) is a proto-language. It is the reconstructed ancestor of the Austronesian languages, one of the world's major language families. Proto-Austronesian is assumed to have begun to diversify ...
. Chickens, together with dogs and pigs, were the domestic animals of the
Lapita culture, the first
Neolithic
The Neolithic period, or New Stone Age, is an Old World archaeological period and the final division of the Stone Age. It saw the Neolithic Revolution, a wide-ranging set of developments that appear to have arisen independently in several p ...
culture of Oceania.
The first pictures of chickens in Europe are found on
Corinthian Corinthian or Corinthians may refer to:
*Several Pauline epistles, books of the New Testament of the Bible:
**First Epistle to the Corinthians
**Second Epistle to the Corinthians
**Third Epistle to the Corinthians (Orthodox)
*A demonym relating to ...
pottery
Pottery is the process and the products of forming vessels and other objects with clay and other ceramic materials, which are fired at high temperatures to give them a hard and durable form. Major types include earthenware, stoneware and ...
of the 7th century BC.
Chickens were spread by
Polynesia
Polynesia () "many" and νῆσος () "island"), to, Polinisia; mi, Porinihia; haw, Polenekia; fj, Polinisia; sm, Polenisia; rar, Porinetia; ty, Pōrīnetia; tvl, Polenisia; tkl, Polenihia (, ) is a subregion of Oceania, made up of ...
n seafarers and reached
Easter Island
Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its ne ...
in the 12th century AD, where they were the only domestic animal, with the possible exception of the
Polynesian rat
The Polynesian rat, Pacific rat or little rat (''Rattus exulans''), known to the Māori as ''kiore'', is the third most widespread species of rat in the world behind the brown rat and black rat. The Polynesian rat originated in Southeast Asia, ...
(''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, Brown Leghorn in Australia
File:Joseph Crawhall - Spanish Cock And Snail.jpg, Joseph Crawhall III
Joseph Crawhall (20 August 1861 – 24 May 1913) was an English artist born in Morpeth, Northumberland.
Life
Crawhall was the fourth child and second son of Joseph Crawhall II and Margaret Boyd. Crawhall specialised in painting animals and b ...
, ''Spanish Cock and Snail''
File:Lavender Orpington Chicken.jpg, Two Lavender Orpington chicks in Ontario
Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...
, Canada
Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...
.
File:More chicks.jpg, A group of chicks
Explanatory notes
References
{{Authority control
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