Gallus gallus domesticus

The chicken (Gallus domesticus ) is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. 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 is a capon. An adult female bird is called a hen and a sexually immature female is called a pullet.

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Originally raised for cockfighting or for special ceremonies, chickens were not kept for food until the Hellenistic period (4th–2nd centuries BC). Humans now keep chickens primarily as a source of food (consuming both their meat and eggs) and as pets.

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

Genetic studies have pointed to multiple maternal origins in South Asia, Southeast Asia, and East Asia, but the clade found in the Americas, Europe, the Middle East and Africa originated from the Indian subcontinent. From ancient India, the chicken spread to Lydia in western Asia Minor, and to Greece by the 5th century BC. Fowl have been known in Egypt since 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.

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Animal name origin

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, and is widely used throughout North America. Roosting is the action of perching aloft to sleep at night.

In Culture

The mythological basilisk or cockatrice is depicted as a reptile-like creature with the upper body of a rooster. Abraxas, a figure in Gnosticism, is portrayed in a similar fashion as well.

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In Greek mythology, Alectryon was a young man that Ares put as a guardian outside his door to inform him if anybody came near while he was making love to Aphrodite, who was married to Hephaestus, Ares' brother. But Alectryon fell asleep while on guard, so Helios, the sun, saw the two lovers and alerted Hephaestus. In anger over Alectryon's incompetence, Ares turned him into a rooster, a bird that always crows at dawn when the sun is about to rise, still loyal to their promise to Ares. The rooster was thus one of Helios' sacred animals.

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Populations of chickens from high altitude regions like Tibet have special physiological adaptations that result in a higher hatching rate in low oxygen environments. When eggs are placed in a hypoxic environment, chicken embryos from these populations express much more hemoglobin than embryos from other chicken populations. This hemoglobin also has a greater affinity for oxygen, allowing hemoglobin to bind to oxygen more readily.

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Pinopsins were originally discovered in the chicken pineal gland.

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Habits and Lifestyle

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.

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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.

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 rooster has only slightly pointed neck feathers, the same colour as the hen's. The identification can be made by looking at the comb, 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 found in several chicken breeds which causes extra feathering under the chicken's face, giving the appearance of a 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.

Social behaviour

Chickens are gregarious birds and live together in flocks. 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', 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.

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.

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Bird's call

Diet and Nutrition

Mating Habits

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.

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More specifically, mating typically involves the following sequence:

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.

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

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

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

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Keeping chickens as pets became increasingly popular in the 2000s among urban and suburban residents. Many people obtain chickens for their egg production but often name them and treat them as any other pet like cats or dogs. Chickens provide companionship and have individual personalities. While many do not cuddle much, they will eat from one's hand, jump onto one's lap, respond to and follow their handlers, as well as show affection.

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

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1. Chicken Wikipedia article -
2. Xeno-canto bird call -

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