Chicken

The word chicken comes from Old English cicen (pronounced essentially the same as in Modern English). Chickens are social animals with complex vocalizations and behaviors, and feature in folklore, religion, and literature across many societies.

Use by humans

Hens of some breeds can produce over 300 eggs per year; the highest authenticated rate of egg-laying is 371 eggs in 364 days. During the Hellenistic period (4th–2nd centuries BC), in the southern Levant, chickens began to be widely domesticated for food. The red junglefowl 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. Chickens are descended primarily from the red junglefowl (Gallus gallus) and are scientifically classified as the same species. Under natural conditions, most birds lay only until a clutch is complete; they then incubate all the eggs.

The chicken is perhaps the most widely domesticated fowl, raised worldwide for its meat and eggs. These domesticated chickens spread across Southeast and South Asia where they interbred with local wild species of junglefowl, forming genetically and geographically distinct groups. Specialized breeds such as broilers and laying hens have been developed for meat and egg production, respectively. 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. More than 50 billion chickens are reared annually as a source of meat and eggs. An early study proposed that a single domestication event of the red junglefowl in present-day Thailand gave rise to the modern chicken.

It is estimated that chickens share between 71 and 79% of their genome with red junglefowl. The domestic chicken has subsequently hybridised with grey junglefowl, Sri Lankan junglefowl and green junglefowl; a gene for yellow skin, for instance, was incorporated into domestic birds from the grey junglefowl (G. sonneratii). Domesticated chickens freely interbreed with populations of red junglefowl.

Domestication and economic production

Chickens are featured widely in folklore, religion, literature, and popular culture. For instance, many important discoveries in limb development have been made using chicken embryos, such as the discovery of the apical ectodermal ridge and the zone of polarizing activity. Equally important, embryologists can carry out experiments on such embryos, close the egg again and study the effects later in development. Cockfighting is outlawed in many countries as involving cruelty to animals.

Chickens are gregarious, living in flocks, and incubate eggs and raise young communally.
The pseudo-riddle “Why did the chicken cross the road?” dates to 1847, or earlier.
Chickens belonging to the same age cohort and sex are often kept together in industrial production settings.
Newly hatched chicks of both modern and heritage varieties weigh the same, about 37 g (1.3 oz).
Adult chickens of both sexes have a fleshy crest on their heads called a comb or cockscomb, and hanging flaps of skin on either side under their beaks called wattles; combs and wattles are more prominent in males.

The chicks imprint on the hen and subsequently follow her continually. Eggs of chickens from the high-altitude region of Tibet have special physiological adaptations that result in a higher hatching rate in low oxygen environments. A flock thus uses only a few preferred locations, rather than having a different nest for every bird. Hens often try to lay in nests that already contain eggs and sometimes move eggs from neighbouring nests into their own.

Hens remain on the nest for about two days after the first chick hatches; during this time the newly hatched chicks feed by absorbing the internal yolk sac. The hen sits on the nest, fluffing up or pecking most play game cricket defensively if disturbed. Reproduction declines with age, thought to be due to a decline in GnRH-I-N.

What do Chickenss eat?

All the time she is sitting in the nest she will regularly turn the eggs keeping them at a constant temperature and humidity. When a hen becomes familiar coming to his ‘call’ the rooster may mate with the hen and fertilize her egg. Removing hens or roosters from a flock causes a temporary disruption to this social order until a new pecking order is established. Chickens have a communal approach to the incubation of eggs and raising of young. Chickens are gregarious birds and live together as a flock.

Certain breeds, such as silkies and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities. 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. 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. Broiler breeds typically take less than six weeks to reach slaughter size, some weeks longer for free-range and organic broilers. Opponents of intensive farming argue that it harms the environment, creates human health risks and is inhumane towards sentient animals. Breeding increased under the Roman Empire and reduced in the Middle Ages.

Wild junglefowl can fly, whereas domestic chickens and their flight muscles are too heavy to allow them to fly more than a short distance. If the eggs are not fertilized and do not hatch, the hen will eventually grow tired of being broody and leave the nest. The hen can hear the chicks peeping inside the eggs, and will gently cluck to encourage them to break out of their shells. Sometimes a hen will stop laying eggs to concentrate on the incubation of her eggs. When a rooster finds food he may call the other chickens to eat it first. Although chickens are flightless birds, they do have a tendency to attempt flight.