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Chicken

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This article is about the animal. For chicken as human food, see Chicken (food). For other uses, see Chicken (disambiguation). ``Gallus gallus domesticus`` redirects here. For other subspecies, see Red Junglefowl. This article is in need of attention from an expert on the subject. WikiProject Birds or the Birds Portal may be able to help recruit one. (September 2009) Chicken A rooster (left) and hen (right) Conservation status Domesticated Scientific classification Kingdom: Animalia Phylum: Chordata Class: Aves Order: Galliformes Family: Phasianidae Genus: Gallus Species: Gallus gallus Subspecies: Gallus gallus domesticus (Linnaeus, 1758) Synonyms

Chicken : Rooster (m), Hen (f)

The chicken (Gallus gallus domesticus) is a domesticated fowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003,[1] there are more chickens in the world than any other bird. Humans keep chickens primarily as a source of food, consuming both their meat and their eggs.

Conventional wisdom has held that the chicken was domesticated in India,[2] but recent evidence suggests that domestication of the chicken was already under way in Vietnam over 10,000 years ago.[2] From India the domesticated fowl made its way to the Persianized kingdom of Lydia in western Asia Minor, domestic fowl were imported to Greece by the fifth century BC.[3] Fowl had been known in Egypt since the 18th Dynasty, with the ``bird that lays every day`` having come to Egypt from the land between Syria and Shinar, Babylonia, according to the annals of Tutmose III.[4]

The chicken is believed to have descended from both the Red Junglefowl (Gallus gallus) and the Grey Junglefowl (G. sonneratii), though hybrids of both wild types usually tend to be sterile.[clarification needed][citation needed] Recent genetic work has revealed that the genotype for yellow skin present in the domestic fowl is not present in what is otherwise its closest kin, the Red Junglefowl. It is most likely that the yellow skin trait in domestic birds originated in the Grey Junglefowl.[5]

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Terminology

In the UK and Canada adult male chickens are known as cocks whereas in America and Australia they are called roosters. Males under a year old are cockerels.[6] Castrated roosters are called capons (though both surgical and chemical castration are now illegal in some parts of the world). Females over a year old are known as hens, and younger females are pullets.[7] In Australia and New Zealand (also sometimes in Britain), there is a generic term chook (pronounced /ˈtʃʊk/: rhymes with ``book``) to describe all ages and both sexes.[8] Babies are called chicks, and the meat is called chicken.

``Chicken`` was originally the word only for chicks, and the species as a whole was then called domestic fowl, or just fowl. This use of ``chicken`` survives in the phrase ``Hen and Chickens``, sometimes used as a British public house or theatre name, and to name groups of one large and many small rocks or islands in the sea (see for example Hen and Chicken Islands).

General biology and habitat

Chickens are omnivores.[9] In the wild, they often scratch at the soil to search for seeds, insects and even larger animals such as lizards or young mice[10].

Chickens may live for five to eleven years, depending on the breed.[11] In commercial intensive farming, a meat chicken generally lives only six weeks before slaughter.[12] A free range or organic meat chicken will usually be slaughtered at about 14 weeks. Hens of special laying breeds may produce as many as 300 eggs a year. After 12 months, the hen's egg-laying ability starts to decline, and commercial laying hens are then slaughtered and used in baby foods, pet foods, pies and other processed foods.[13] The world's oldest chicken, according to the Guinness Book of World Records, died of heart failure when she was 16 years old.[14]

The adult rooster can be distinguished from the hen by its comb

Roosters can usually be differentiated from hens by their striking plumage, marked by long flowing tails and shiny, pointed feathers on their necks and backs (the hackles and saddle)—these are often colored differently from the hackles and saddles of females.

However, in some breeds, such as the Sebright, the cock has only slightly pointed neck feathers, the same colour as the hen's. The identification must 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. 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.

Wild Red Junglefowl- Male at 23 Mile near Jayanti in Buxa Tiger Reserve in Jalpaiguri district of West Bengal, India.

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

Chickens are gregarious birds and live together as a flock. 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 access to food 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 violence and injury.[15]

Hens will try to lay in nests that already contain eggs, and have been known to move eggs from neighbouring nests into their own. Some farmers use fake eggs made from plastic or stone (or golf balls) to encourage hens to lay in a particular location. The result of this behavior is that a flock will use only a few preferred locations, rather than having a different nest for every bird.

Hens can also be extremely stubborn about always laying 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.

Skull of a chicken three weeks old. Here the opisthotic bone appears in the occipital region, as in the adult Chelonian. bo = Basi-occipital, bt = Basi-temporal, eo = Opisthotic, f = Frontal, fm = Foramen magnum, fo = Fontanella, oc = Occipital condyle, op = Opisthotic, p = Parietal, pf = Post-frontal, sc = Sinus canal in supra-occipital, so = Supra-occpital, sq = Squamosal, 8 = Exit of vagus nerve.

Roosters crowing (a loud and sometimes shrill call) is a territorial signal to other roosters. However, crowing may also result from sudden disturbances within their surroundings. Hens cluck loudly after laying an egg, and also to call their chicks.

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...``[16]

Courting

When a rooster finds food, he may call the other chickens to eat it first. He does this by clucking in a high pitch as well as picking up and dropping the food. This behavior can also be observed in mother hens, calling their chicks. In some cases the rooster will drag the wing opposite the hen on the ground, while circling her. This is part of chicken courting ritual and has been called a ``dance``.[17] The dance triggers a response in the hen's brain,[17] and when the hen responds to his ``call``, the rooster may mount the hen and proceed with the fertilization.

Breeding

Origins

Formerly, phenotypic diversity of modern chickens led to a belief of polyphyletic origins.[18] According to genetic researchers, all modern chicken genes can be derived from the subspecies of Gallus found in northeast Thailand.[19][20] This is supported by archaeological findings. Researchers have found chickens' bones in unusual amounts and out of natural jungle range, thus denoting a breeding place. Bones of domestic chickens have been found about 6000-4000 BC in Yangshao and Peiligan, China, while the Holocene climate was not naturally suitable for the Gallus species.[21] Archaeological data is lacking for Thailand and southeast Asia.

Later traces are found about 3000-2000 BC in Harappa and Mohenjo-Daro, Pakistan,[22] and -according to linguistic researchers- in Austronesian populations traveling across southeast Asia and Oceania. A northern road spread chicken to the Tarim basin of central Asia, modern day Iran. The chicken reached Europe (Romania, Turkey, Greece, Urkraine) about 3000BC, and the Indus Valley about 2500 BC.[19] Introduction into Western Europe came far later, about the 1st millennium BC. Phoenicians spread chickens along the Mediterranean coasts, to Iberia. Breeding increased under the Roman Empire, and was reduced in the Middle Ages.[19] Middle East traces of chicken go back to a little earlier than 2000 BC, in Syria; chicken went southward only in the 1st millennium BC. The chicken reached Egypt for purposes of cock fighting about 1400BC, and became widely bred only in Ptolemaic Egypt (about 300 BC).[19] Little is known about the chicken's introduction into Africa. Three possible ways of introduction in about the early first millennium AD could have been through the Egyptian Nile 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.[19] Domestic chicken in the Americas before Western conquest is still an ongoing discussion, but blue-egged chicken, found only in the Americas and Asia, suggest an Asian origin for early American chickens.[19]

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.[19]

Current

Chicken eggs vary in color depending on the hen, typically ranging from bright white to shades of brown and even blue, green, and recently reported purple (found in South Asia) (Araucana varieties).

Under natural conditions, most birds lay only until a clutch is complete, and they will then incubate all the eggs. Many domestic hens will also do this–and 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, protesting or pecking in defense if disturbed or removed, and she 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, an owner may place many artificial eggs in the nest, or to stop it they may place the hen in an elevated cage with an open wire floor.

At the end of the incubation period (about 21 days),[17] the eggs, if fertile, will hatch. Development of the egg starts only when incubation begins, so they all 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. It will then rest 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). It then enlarges the hole, gradually turning round as it goes, and eventually severing the blunt end of the shell completely to make a lid. It crawls out of the remaining shell, and its wet down dries out in the warmth of the nest.

A day-old chick

The hen will usually stay on the nest for about two days after the first egg hatches, and during this time the newly-hatched chicks live off the egg yolk they absorb just before hatching. Any eggs not fertilized by a rooster will not hatch, and the hen eventually loses interest in these and leaves the nest. After hatching, the hen fiercely guards the chicks, and will brood them when necessary to keep them warm, at first often returning to the nest at night. She leads them to food and water; she will call them to edible items, but seldom feeds them directly. She continues to care for them until they are several weeks old, when she will gradually lose interest and eventually start to lay again.

Modern egg-laying breeds rarely go broody, and those that do often stop part-way through the incubation. However, some ``utility`` (general purpose) breeds, such as the Cochin, Cornish and Silkie, do regularly go broody, and they 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, pheasants, turkeys or geese. Chicken eggs can also be hatched under a broody duck, with varied success.

Poultry farming

Main article: Poultry farming A free range rooster on a farm

More than 50 billion chickens are reared annually as a source of food, for both their meat and their eggs. Chickens farmed for meat are called broiler chickens, whilst those farmed for eggs are called egg-laying hens. In total, the UK alone consumes over 29 million eggs per day. Some hens can produce over 300 eggs a year. Chickens will naturally live for 6 or more years, but broiler chickens typically take less than six weeks to reach slaughter size. For laying hens, they are slaughtered after about 12 months, when the hens' productivity starts to decline.

The vast majority of poultry are raised using intensive farming techniques. According to the Worldwatch Institute, 74 percent of the world's poultry meat, and 68 percent of eggs are produced this way.[3] One 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 due to increased productivity, stating that the animals are looked after in state-of-the-art environmentally controlled facilities.

Artificial incubation

An egg incubator.

Chicken egg incubation can successfully occur artificially as well. Nearly all fertilized chicken eggs will hatch after 21 days of good conditions - 99.5 °F (37.5 °C) and around 55% relative humidity (increase to 70% in the last three days of incubation to help soften egg shell). Eggs must be turned regularly (usually three to eight times each week) during the first part of the incubation. If the eggs aren't turned, the embryo inside will stick to the shell and may hatch with physical defects. Some incubators turn the eggs automatically. This turning mimics the natural process. An incubating hen will stand up several times a day and shift the eggs around with her beak. However, if the egg is turned during the last week of incubation the chick may have difficulty settling in the correct hatching position.

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 half a dozen to 75 eggs; they are usually electrically powered, but in the past some were heated with an oil or paraffin lamp.

Chickens as food

Main article: Chicken (food) Roasted chicken.

The meat of the chicken, also called ``chicken``, is a type of poultry meat. Because of its relatively low cost, chicken is one of the most used meats in the world. Nearly all parts of the bird can be used for food, and the meat can be cooked in many different ways. Popular chicken dishes include roasted chicken, fried chicken, chicken soup, Buffalo wings, tandoori chicken, butter chicken, and chicken rice. Chicken is also a staple of fast food restaurants.

Chickens as pets

Main article: Chickens as pets

Chickens are sometimes kept as pets and can be tamed by hand feeding, but roosters can sometimes become aggressive and noisy. Some have advised against keeping them around very young children. Certain breeds, however, such as silkies and many bantam varieties are generally docile and are often recommended as good pets around children.[23] Some people find chickens' behaviour entertaining and educational.[24]

Baby chickens, dyed unnatural colors, sold as pets at a Market in Oaxaca, Mexico.

While some cities in the United States allow chickens as pets, the practice is not approved in all localities. Some communities ban only roosters, allowing the quieter hens. The so called ``urban hen movement`` harks back to the days when chicken keeping was much more common, and involves the keeping of small groups of hens in areas where they may not be expected, such as closely populated cities and suburban areas. City ordinances, zoning regulations or health boards may determine whether chickens may be kept.[25] A general requirement is that the birds be confined to the owner's property, not allowed to roam freely. There may be strictures on the size of the property or how far from human dwellings a coop may be located, etc.[26]

In Asia, chickens with striking plumage have long been kept for ornamental purposes, including feather-footed varieties such as the Cochin from Vietnam, the Silkie from China, and the extremely long-tailed Phoenix from Japan. Asian ornamental varieties were imported into the United States and Great Britain in the late 1800s. Distinctive American varieties of chickens have been developed from these Asian breeds. Poultry fanciers began keeping these ornamental birds for exhibition, a practice that continues today. Individuals in rural communities commonly keep chickens for both ornamental and practical value.

Chicken diseases and ailments

Chickens are susceptible to several parasites, including lice, mites, ticks, fleas, and intestinal worms, as well as other diseases. (Despite the name, they are not affected by Chickenpox; the illness is generally restricted to humans.[27])

Some of the common diseases that affect chickens are shown below:

Name Common Name Caused by Aspergillosis fungi Avian influenza bird flu virus Histomoniasis Blackhead disease protozoal parasite Botulism toxin Cage Layer Fatigue mineral deficiencies, lack of exercise Campylobacteriosis tissue injury in the gut Coccidiosis parasites Colds virus Crop Bound improper feeding Dermanyssus gallinae Red mite parasite Egg bound oversized egg Erysipelas bacteria Fatty Liver Hemorrhagic Syndrome high-energy food Fowl Cholera bacteria Fowl pox virus Fowl Typhoid bacteria Gallid herpesvirus 1 or Infectious Laryngotracheitis virus Gapeworm Syngamus trachea worms Infectious Bronchitis virus Infectious Bursal Disease Gumboro virus Infectious Coryza bacteria Lymphoid leukosis Avian leukosis virus