chicken

bird
Also known as: Gallus domesticus

chicken, (Gallus gallus), any of more than 60 breeds of medium-sized poultry that are primarily descended from the wild red jungle fowl (Gallus gallus, family Phasianidae, order Galliformes) of India. The chicken is perhaps the most widely domesticated fowl, raised worldwide for its meat and eggs.

Despite the chicken’s close relationship with the red jungle fowl, there is evidence that the gray jungle fowl (G. sonneratii) of southern India and other jungle fowl species, also members of Gallus, may have contributed to the bird’s ancestry. There is some debate about what the chicken’s scientific name should be. Although many taxonomists and ornithologists consider it as a domesticated form of the wild red jungle fowl, some classify it as a subspecies of the red jungle fowl (i.e., G. gallus domesticus), whereas others, including the U.S. Department of Agriculture, classify the bird as G. domesticus.

Natural history

Chickens have a squat and rounded appearance. They stand less than 70 cm (27.6 inches) tall and weigh approximately 2.6 kg (5.7 pounds) on average. Males (called cocks or roosters) and females (hens) are known for their fleshy combs, lobed wattles hanging below the bill, and high-arched tails. In some roosters, the tail can extend more than 30 cm (12 inches) in length.

Lion (panthera leo)
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Chickens breed in the spring and summer months. Egg laying is stimulated by the long stretches of daylight that occur during the warmer months; however, artificial lights placed in chicken coops can trigger a hen’s egg laying response throughout the year. The time between ovulation and egg-laying is approximately 23–26 hours. Subsequent ovulations may occur within an hour after the previous egg was laid, allowing some hens to produce as many as 300 eggs per year.

Fertilized embryos develop quickly, and chicks hatch approximately 21 days later. Chicks are born covered in down, but they mature quickly, becoming fully feathered after four to five weeks. At about six months, males produce viable sperm, and females produce viable eggs. Members of free-ranging flocks may live for six to eight years under the best conditions, but most chickens used in the poultry industry serve as egg layers for two to three years before being slaughtered for their meat, with much of it being used in pet food. Chickens in captivity have been known to live for up to 30 years.

Social hierarchy

Each flock of chickens develops a social hierarchy that determines access to food, nesting sites, mates, and other resources. A flock usually includes one dominant adult male, a few subdominant males, and two or more females that are carefully watched over by the dominant male. Social hierarchies in chickens are segregated by sex and manifest as a pecking order, in which individuals of higher social rank may strike out at individuals of lower rank with their beaks (pecking) to ensure access to food and other resources. Altercations, however, may also include pummeling with wings and scratching with claws.

Chickens belonging to the same age cohort and sex are often kept together in industrial production settings. The pecking order is established within groups of female chicks by the 10th week of life. In groups of male chicks, however, fights for dominance may continue into adulthood. In situations where one adult bird challenges another—which happens most often when a new bird is introduced into the flock—fights involving males risk injury and death more often than fights involving females.

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Domestication and economic production

Chicken domestication likely occurred more than once in Southeast Asia and possibly India over the most recent 7,400 years, and the first domestications may have been for religious reasons or for the raising of fighting birds. Descendants of those domestications have spread throughout the world in several waves for at least the last 2,000 years. For most of that period, chickens were a common part of the livestock complement of farms and ranches throughout Eurasia and Africa. Only in the early 20th century, however, did chicken meat and eggs become mass-production commodities.

Modern high-volume poultry farms, with rows of cages stacked indoors for control of heat, light, and humidity, began to proliferate in Great Britain about 1920 and in the United States after World War II. Females (mature hens and younger chickens, called pullets) are raised for meat and for their edible eggs. Farmers have developed numerous breeds and varieties to fulfill commercial requirements.

Originally, meat production was a by-product of egg production. Only hens that could no longer produce enough eggs were killed and sold for meat. By the mid-20th century, however, meat production had outstripped egg production as a specialized industry. The market for chicken meat has grown dramatically since then, with worldwide exports reaching nearly 12.5 million metric tons (about 13.8 million tons) by the early 21st century.

Mature males have long been used for sport (i.e., cockfighting, now outlawed in many jurisdictions) as well as for breeding. Many immature males (cockerels) are castrated (usually chemically, with hormones that cause atrophying of the testicles) to become meat birds (capons).

The Editors of Encyclopaedia Britannica
This article was most recently revised and updated by John P. Rafferty.

poultry farming, raising of birds domestically or commercially, primarily for meat and eggs but also for feathers. Chickens, turkeys, ducks, and geese are of primary importance, while guinea fowl and squabs (young pigeons) are chiefly of local interest. This article treats the principles and practices of poultry farming. For a discussion of the food value and processing of poultry products, see egg and poultry processing.

Commercial production

Feeding

Commercial poultry feeding is a highly perfected science that ensures a maximum intake of energy for growth and fat production. High-quality and well-balanced protein sources produce a maximum amount of muscle, organ, skin, and feather growth. The essential minerals produce bones and eggs, with about 3 to 4 percent of the live bird being composed of minerals and 10 percent of the egg. Calcium, phosphorus, sodium, chlorine, potassium, sulfur, manganese, iron, copper, cobalt, magnesium, and zinc are all required. Vitamins A, C, D, E, and K and all of the B vitamins are also required. Antibiotics are widely used to stimulate appetite, control harmful bacteria, and prevent disease. For chickens, modern rations produce about 0.5 kg (1 pound) of broiler on about 0.9 kg (2 pounds) of feed and a dozen eggs from 2 kg (4.5 pounds) of feed.

Management

A carefully controlled environment that avoids crowding, chilling, overheating, or frightening is almost universal in poultry farming. Cannibalism, which expresses itself as toe picking, feather picking, and tail picking, is controlled by debeaking at one day of age and by other management practices. The feeding, watering, egg gathering, and cleaning operations are highly mechanized. Birds are usually housed in wire cages with two or three animals per cage, depending on the species and breed, and three or four tiers of cages superposed to save space. Cages for egg-laying birds have been found to increase production, lower mortality, reduce cannibalism, lower feeding requirements, reduce diseases and parasites, improve culling, and reduce both space and labour requirements.

Poultry breeding is an outstanding example of the application of basic genetic principles of inbreeding and crossbreeding as well as of intensive mass selection to effect faster and cheaper gains in meat and maximum egg production for the egg-laying strains. Maximum use of heterosis, or hybrid vigour, through incrosses and crossbreeding has been made. Rapid and efficient weight gains and high-quality, plump, meaty carcasses have been achieved thereby.

Among the world’s agricultural industries, chicken breeding in the U.S. is one of the most advanced. Intensive nutritional research and application, highly improved breeding stock, intelligent management, and scientific disease control have gone into the effort to give a modern broiler (meat chicken) of uniformly high quality produced at ever-lower cost. A modern broiler chick can reach a 2.3-kg (5-pound) market weight in five weeks, compared with the four months that were required in the mid-20th century. Additionally, annual egg production per hen has increased from about 100 in 1910 to over 300 in the early 21st century.

Diseases

Poultry are quite susceptible to a number of diseases. Some of the more common are fowl typhoid, pullorum, fowl cholera, chronic respiratory disease, infectious sinusitis, infectious coryza, avian infectious hepatitis, infectious synovitis, bluecomb, Newcastle disease, fowl pox, avian leukosis complex, coccidiosis, blackhead, infectious laryngotracheitis, infectious bronchitis, and erysipelas. Strict sanitary precautions, the intelligent use of antibiotics and vaccines, and the widespread use of cages for layers and confinement rearing for broilers have made it possible to effect satisfactory disease control.

Outbreaks of bird flu, or avian influenza, which was first detected in humans in 1997, have led to the culling of millions of poultry animals since the late 20th century. Waterfowl such as wild ducks are thought to be primary hosts for all bird flu subtypes. Though normally resistant to the viruses, the birds carry them in their intestines and distribute them through feces into the environment, where they infect susceptible domestic birds. Sick birds pass the viruses to healthy birds through saliva, nasal secretions, and feces. Within a single region, bird flu is transmitted readily from farm to farm by airborne feces-contaminated dust and soil, by contaminated clothing, feed, and equipment, or by wild animals carrying the virus on their bodies. The disease is spread from region to region by migratory birds and through international trade in live poultry. Humans who are in close contact with sick birds—for example, poultry farmers and slaughterhouse workers—are at the greatest risk of becoming infected.

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Parasitic diseases of poultry, including hexamitiasis of turkeys, are caused by roundworms, tapeworms, lice, and mites. Again, modern methods of sanitation, prevention, and treatment provide excellent control.