Chicken Murgha

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 different warning calls when they sense a predator approaching from the air or on the ground.



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.

More specifically, mating typically involves the following sequence: 1. Male approaching the hen. 2. Male pre-copulatory waltzing. 3. Male waltzing. 4. Female crouching (receptive posture) or stepping aside or running away (if unwilling to copulate). 5. Male mounting. 6. Male treading with both feet on hen’s back. 7. Male tail bending (following successful copulation).

Nesting and laying behaviour

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).

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.

Broodiness
Under natural conditions, most birds lay only until a clutch is complete, and they will then incubate all the eggs. Hens 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.


Skull of a three-week-old chicken. 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.
Breeds artificially developed for egg production rarely go broody, and those that do often stop part-way through the incubation. However, other 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.

Hatching and early life
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 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.

Defensive behaviour
Chickens may occasionally gang up on a weak or inexperienced predator. At least one credible report exists of a young fox killed by hens.
Embryology
File:Embryo.ogv
Earliest gestation stages and blood circulation of a chicken embryo
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..

Breeding
Origins

Male red junglefowl.
Galliformes, the class of bird that chickens belong to, is directly linked to the survival of birds when all other dinosaurs went extinct. It was water or ground-dwelling fowl much like modern partridges that survived the fireball wiping out all tree-dwelling birds with the rest of the dinosaurs. Some of these evolved into the modern galliformes, of which domesticated chickens are a main model. They are descended primarily from the red junglefowl (Gallus gallus) and are scientifically classified as the same species. As such, they can and do freely interbreed with populations of red junglefowl. 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).

The traditional view is that chickens were first domesticated for cockfighting in Asia, Africa, and Europe.  In the last decade, there have been a number of genetic studies to clarify the origins. According to one early study, a single domestication event which took place in what now is the country of Thailand gave rise to the modern chicken with minor transitions separating the modern breeds. However, that study was later found to be based on incomplete data, and recent studies point to multiple maternal origins, with the clade found in the Americas, Europe, Middle East, and Africa, originating from the Indian subcontinent, where a large number of unique haplotypes occur. It is postulated that the red junglefowl, known as the bamboo fowl in many Southeast Asian languages, is a special bird well-adapted to take advantage of the large amounts of fruits that are produced during the end of the 50-year bamboo seeding cycle, to boost its own reproduction. In domesticating the chicken, humans took advantage of this predisposition for prolific reproduction of the red junglefowl when exposed to large amounts of food.

It has been claimed (based on paleoclimatic assumptions) 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. Introduction into Western Europe came far later, about the 1st millennium BC. Phoenicians spread chickens along the Mediterranean coasts as far as Iberia. Breeding increased under the Roman Empire, and was reduced in the Middle Ages. Genetic sequencing of chicken bones from archaeological sites in Europe revealed that in the High Middle Ages chickens became less aggressive and began to lay eggs earlier in the breeding season.

Middle East traces of chicken go back to a little earlier than 2000 BC, in Syria; chickens went southward only in the 1st millennium BC. They reached Egypt for purposes of cockfighting about 1400 BC, and became widely bred only in Ptolemaic Egypt (about 300 BC). Little is known about the chicken's introduction into Africa. It was during the Hellenistic period (4th-2nd centuries BC), in the Southern Levant, that chickens began widely to be domesticated for food.  This change occurred at least 100 years before domestication of chickens spread to Europe.

Three possible routes 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 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.

South America
An unusual variety of chicken that has its origin

Use by humans
Farming
Main articles: Poultry farming and Chicken as food

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 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.[49]

The vast majority of poultry are 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.[50] An 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.[51] 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.[52]

Reared for meat
Main article: Broiler

A commercial chicken house with open sides raising broiler pullets for meat
Chickens farmed for meat are called broilers. Chickens will naturally live for six or more years, but broiler breeds typically take less than six weeks to reach slaughter size.[53] A free range or organic broiler will usually be slaughtered at about 14 weeks of age.

Reared for eggs
Main article: Egg as food
Chickens farmed primarily for eggs are called layer hens. In total, the UK alone consumes more than 34 million eggs per day.[54] Some hen breeds can produce over 300 eggs per year, with "the highest authenticated rate of egg laying being 371 eggs in 364 days".[55] 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.[56] In the UK and Europe, laying hens are then slaughtered and used in processed foods or sold as "soup hens".[56] 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[57] or sufficiently long to cause a body weight loss of 25 to 35%,[58] or up to 28 days under experimental conditions.[59] 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.[60]

As pets
Main article: Chickens as pets
Chickens are social, inquisitive, intelligent birds, and many find their behaviour entertaining.[61] Certain breeds, such as Silkies and many bantam varieties, are generally docile and are often recommended as good pets around children with disabilities.[62]



Artificial incubation

An egg incubator
Incubation can successfully occur artificially in machines that provide the correct, controlled environment for the developing chick.[63][64] 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.

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, which is generally restricted to humans.[65]

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