A fresh Chicken meat
A rooster (left) and hen (right)
Conservation status
Gay
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Galliformes
Family: Phasianidae
Genus: Gallus
Species: G. gallus

A chicken (Gallus gallus domesticus) is a kind of domesticated bird. It is raised widely for its meat and eggs. There are 24 billion chickens worldwide. They are usually kept by humans as livestock, which means it is kept for its meat and eggs. Most breeds of chickens can fly for short distances and some roost in trees if these are provided.

A male chicken is called a rooster or a cock (short for cockerel). A female chicken is called a hen. A young chicken is called a chick. Like other female birds, hens lay eggs which can hatch into chicks.

When raising chickens, a farmer needs to build a chicken coop (like a little house) for the chickens to roost (sleep) in. They also need a run or yard where they can exercise, take dust baths, eat, and drink. The chickens also need to be protected from predators such as foxes, which is often done with fences.

Chickens can also be farmed intensively. This lets farms make a lot of chicken meat, and eggs, but this is not as good for the chickens.

General biology and habitat

Rooster portrait2
In some breeds the adult rooster can be distinguished from the hen by his larger comb
Didactic model of a chicken--FMVZ USP-29
Anatomy of a chicken.

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 young mice.

Chickens may live for five to ten years, depending on the breed. The world's oldest chicken was a hen which died of heart failure at the age of 16 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 birds 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

Hen with chicks, Raisen district, MP, India
Hen with chicks, India.
Cascais Costa do Esteril 52 (36583204550)
Hen with chicks, Portugal.

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. 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 a low "warning call" when they sense a predator approaching.

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.

Nesting and laying behaviour

Chicken eggs
Chicken eggs vary in colour 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).
Küken vor dem ersten Ausflug
Chicks before their first outing

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. Some farmers use fake eggs made from plastic or stone (or golf balls) to encourage hens to lay in a particular location.

Broodiness

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, 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. 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.

Hatching and early life

Oeuf en train d'eclore
Chickens hatching from eggs

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 commercial 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.

Breeding

Origins

Gallus gallus male Kaziranga 0
Male red junglefowl.

The domestic chicken is descended primarily from the red junglefowl (Gallus gallus) and is scientifically classified as the same species. As such it can and does freely interbreed with populations of red jungle fowl. Recent genetic analysis has revealed that at least the gene for yellow skin was incorporated into domestic birds through hybridization with the grey junglefowl (G. sonneratii).

Recent studies show chickens were found in the Americas, Europe, Middle East, and Africa, originating from the Indian subcontinent, where a large number of unique haplotypes occur.

It has been claimed that chickens were domesticated in Southern China in 6000 BC. However a recent study, raises doubts as to whether those birds were the ancestors of chickens today. Instead, the origin could be the Harappan culture of the Indus Valley. Eventually, the chicken moved to the Tarim basin of central Asia. The chicken reached Europe (Romania, Turkey, Greece, Ukraine) about 3000 BC.

Middle East traces of chicken go back to a little earlier than 2000 BC, in Syria.

Domestic chicken in the Americas before Western conquest is still an ongoing discussion, but blue-egged chickens, found only in the Americas and Asia, suggest an Asian origin for early American chickens.

A lack of data from Thailand, Russia, the Indian subcontinent, Southeast Asia and Sub-Saharan Africa makes it difficult to lay out a clear map of the spread of chickens in these areas; better description and genetic analysis of local breeds threatened by extinction may also help with research into this area.

Araucana hen showing ear tufts
Araucana hen showing ear tufts

South America

An unusual variety of chicken that has its origins in South America is the araucana, bred in southern Chile by the Mapuche people. Araucanas, some of which are tailless and some of which have tufts of feathers around their ears, lay blue-green eggs.

It has long been suggested that they pre-date the arrival of European chickens brought by the Spanish and are evidence of pre-Columbian trans-Pacific contacts between Asian or Pacific Oceanic peoples, particularly the Polynesians, and South America. Radiocarbon dating results in 2007 appeared to confirm that the chickens came from Polynesia and that there were contacts between Polynesia and South America before Columbus's arrival in the Americas.

Farming

BatteryChicken5DaysOutOfCage
A former battery hen, five days after release. Note the pale comb - the comb may be a reliable indicator of health or vigor.

More than 50 billion chickens are reared annually as a source of meat and eggs. In the US 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 is raised in factory farms. According to the Worldwatch Institute, 74 percent of the world's poultry meat and 68 percent of eggs are produced this way. 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 owing to increased productivity, and that the animals are looked after in state-of-the-art environmentally controlled facilities.

In part due to the conditions on intensive poultry farms and recent recalls of large quantities of eggs, there is a growing movement for small-scale micro-flocks or 'backyard chickens'. This involves keeping small numbers of hens (usually no more than a dozen) in suburban or urban residential areas to control bugs, to utilize chicken waste as fertilizer in small gardens, and for the high-quality eggs and meat that are produced.

Reared for meat

Florida chicken house
A commercial chicken house with open sides raising broiler pullets for meat

Chickens farmed for meat are called broiler chickens. Chickens will naturally live for six or more years, but broilers typically take less than six weeks to reach slaughter size. A free range 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 can produce over 300 eggs per year, with "the highest authenticated rate of egg laying being 371 eggs in 364 days". After 12 months of laying, the commercial hen's egg-laying ability starts to decline to the point where the flock is commercially unviable. Hens, particularly from battery cage systems, are sometimes infirm or have lost a significant amount of their feathers, and their life expectancy has been reduced from around seven years to less than two years. In the UK and Europe, laying hens are then slaughtered and used in processed foods or sold as "soup hens". In some other countries, flocks are sometimes 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.

Artificial incubation

Egg incubator
An egg incubator

Incubation can successfully occur artificially in machines that provide the correct, controlled environment for the developing chick. The average incubation period for chickens is 21 days but may depend on the temperature and humidity in the incubator. Temperature regulation is the most critical factor for a successful hatch. Variations of more than 1 °C (1.8 °F) from the optimum temperature of 37.5 °C (99.5 °F) will reduce hatch rates. Humidity is also important because the rate at which eggs lose water by evaporation depends on the ambient relative humidity. Evaporation can be assessed by candling, to view the size of the air sac, or by measuring weight loss. Relative humidity should be increased to around 70% in the last three days of incubation to keep the membrane around the hatching chick from drying out after the chick cracks the shell. Lower humidity is usual in the first 18 days to ensure adequate evaporation. The position of the eggs in the incubator can also influence hatch rates. For best results, eggs should be placed with the pointed ends down and turned regularly (at least three times per day) until one to three days before hatching. If the eggs aren't turned, the embryo inside may stick to the shell and may hatch with physical defects. Adequate ventilation is necessary to provide the embryo with oxygen. Older eggs require increased ventilation.

Many commercial incubators are industrial-sized with shelves holding tens of thousands of eggs at a time, with rotation of the eggs a fully automated process. Home incubators are boxes holding from 6 to 75 eggs; they are usually electrically powered, but in the past some were heated with an oil or paraffin lamp.

As food

Rosemary chicken
Oven-roasted rosemary and lemon 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 many fast food restaurants.

Eggs

Eggs Benedict
Eggs Benedict

In 2000, there were 50.4 million tons of eggs produced in the world (Executive guide to world poultry trends, 2001) and an estimated 53.4 million tons of table eggs were produced during 2002. In 2009, an estimated 62.1 million metric tons of eggs were produced worldwide from a total laying flock of approximately 6.4 billion hens.

Chicken eggs are widely used in many types of dishes, both sweet and savory, including many baked goods. Eggs can be scrambled, fried, hard-boiled, soft-boiled, pickled, and poached. The albumen, or egg white, contains protein but little or no fat, and can be used in cooking separately from the yolk. Egg whites may be aerated or whipped to a light, fluffy consistency and are often used in desserts such as meringues and mousse. Ground egg shells are sometimes used as a food additive to deliver calcium. Hens do not need a male to produce eggs, only to fertilize them. A flock containing only females will still produce eggs; however, the eggs will all be infertile.

As pets

Betty up to her hijinks
Pet chickens.

Some people find chickens' behaviour entertaining and educational and may keep them as pets. Chickens can be tamed by hand feeding, although roosters can sometimes become aggressive and noisy. Aggression may be curbed with proper handling. Some have advised against keeping them around very young children. Certain breeds, such as silkies and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities.

Chicken and chicken pox

Chickenpox has nothing to do with chickens. When chicken pox was first described, it was noted that the pox spots looked like the vegetable called chickpeas placed upon the skin. The Latin word for chick peas is cicer, and that is the original word that chicken pox got its name from.

Images


Chicken for Kids. Kiddle Encyclopedia.