Chicken

About the chicken

The chicken is the domesticated form of the red junglefowl, a bird originally native to Southeast Asia. Domestication happened around 8,000 years ago, and the chicken has since become among the most common and widespread domesticated animals on Earth. As of 2023, the global chicken population exceeded 26.5 billion, with more than 50 billion birds produced annually for consumption — a scale that makes the species a central part of global animal husbandry.

Chickens are relatively large, round-bodied birds that are active during the day. Their legs are unfeathered in most breeds, and their wings are short; while their wild junglefowl ancestors can fly, domestic chickens carry flight muscles too heavy to manage more than a short hop or glide. Newly hatched chicks of both modern and heritage varieties weigh about the same, roughly 37 g (1.3 oz), but modern breeds grow much faster — a Ross 708 broiler can reach 1.8 kg (4.0 lb) by day 35, compared with about 1.05 kg (2.3 lb) for a heritage chicken of the same age. Adult chickens of both sexes carry a fleshy comb on the head and hanging wattles of skin under the beak, both more prominent in males. Size and coloration vary enormously between breeds, a product of thousands of years of selective breeding for meat, eggs, and appearance alike.

Genetically, chickens are classified as the same species as the red junglefowl and readily interbreed with wild junglefowl populations where their ranges meet. Domestic chickens have also hybridized with grey, Sri Lankan, and green junglefowl over the course of their spread — a gene for yellow skin, for example, came into domestic birds from the grey junglefowl. Chickens share between 71 and 79 percent of their genome with the red junglefowl, reflecting how recent domestication was in evolutionary terms.

Behavior and diet

Chickens are gregarious birds that live in flocks, incubate eggs, and raise young communally. Within a flock, individuals establish a "pecking order" — a term coined in 1921 after the dominance behavior researchers observed in female chickens — in which more dominant birds get first access to food and nesting sites. Males tend to settle disputes by leaping and using their claws, while a rooster's crow serves as both a territorial signal to other males and a response to sudden disturbances nearby. Hens cluck loudly after laying an egg and to call their chicks, and the species gives distinct warning calls depending on whether a predator is approaching from the air or the ground. Chickens are even capable of mobbing and killing a weak or inexperienced predator, such as a young fox.

As omnivores, wild and free-ranging chickens scratch at the soil in search of seeds and insects, and will take animals as large as lizards, small snakes, and young mice. Courtship involves the rooster performing a "waltzing display" and sometimes a circling dance near the hen; if she is receptive, mating follows, including a distinctive tail-bending display from the male afterward. Hens often lay in nests that already hold eggs, sometimes moving eggs from a neighbor's nest into their own, so a flock tends to use only a handful of preferred nest sites.

Under natural conditions, a hen lays until her clutch is complete, then goes "broody," sitting on the eggs and rarely leaving the nest until they hatch — a process that takes about 21 days, with the chick using an egg tooth to break out of the shell. She remains on the nest for roughly two days after the first chick hatches, while the newly hatched chicks live off their internal yolk sac, then leads and guards her brood, calling them to food, for several weeks.

Range and habitat

The chicken's ancestor, the red junglefowl, is native to Southeast Asia, and archaeological evidence places early domestication in that region — with likely domestic chickens in Southeast Asia well before 6000 BC, in China by 6000 BC, and in India by 2000 BC. From there, the species spread widely: Austronesian peoples carried chickens throughout Island Southeast Asia, Micronesia, Melanesia, Polynesia, and Madagascar starting at least 3000 BC, and the birds reached Egypt via the Middle East by around 1400 BC before becoming widely bred there by 300 BC. Today, chickens are kept essentially everywhere people raise domestic animals, from small backyard flocks to large-scale commercial operations, and the species has expanded well beyond its tropical origins into virtually every climate humans inhabit.

Conservation status

Domestic animals are not assessed on the IUCN Red List, so the chicken has no Red List category. It is a domesticated form of the wild red junglefowl, which remains a separate wild bird.

The pressures on chickens are practical health ones. They are susceptible to parasites such as Dermanyssus gallinae mites, which feed on blood, reduce egg production, and can spread bacterial diseases including salmonellosis, as well as to viral diseases such as avian influenza.