General biology and habitat
Chickens are omnivores. In the wild, they often scratch at the soil to search for seeds, insects and even larger animals such as lizards or young mice.
Chickens may live for five to eleven years, depending on the breed. In commercial intensive farming, a meat chicken generally lives only six weeks before slaughter. A free range or organic meat chicken will usually be slaughtered at about 4 weeks. Hens of special laying breeds may produce as many as 300 eggs a year. After 2 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. The world's oldest chicken, according to the Guinness Book of World Records, died of heart failure when she was 6 years old.
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.
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.
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.
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...
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." The dance triggers a response in the hen's brain, and when the hen responds to his "call," the rooster may mount the hen and proceed with the fertilization.
Formerly, phenotypic diversity of modern chickens led to a belief of polyphyletic origins. According to genetic researchers, all modern chicken genes can be derived from the subspecies of Gallus found in northeast Thailand.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. Archaeological data is lacking for Thailand and southeast Asia.
Later traces are found about 3000-2000 BC in Hrappa and Mohenjo-Daro, Pakistan, 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.Introduction into Western Europe came far later, about the st millennium BC. Phoenicians spread chickens along the Mediterranean coasts, to Iberia. Breeding increased under the Roman Empire, and was reduced in the Middle Ages.Middle East traces of chicken go back to a little earlier than 2000 BC, in Syria; chicken went southward only in the st millennium BC. The chicken reached Egypt for purposes of cock fighting about 400BC, and became widely bred only in Ptolemaic Egypt (about 300 BC). 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. 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.
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.