Chicken

The chicken (Gallus gallus domesticus) is a domesticated form of the red junglefowl, originally native to Southeast Asia. It was first domesticated around 8,000 years ago and is one of the most common and widespread domesticated animals in the world. Chickens are primarily kept for their meat and eggs, though they are also kept as pets.

As of 2023, the global chicken population exceeds 26.5 billion, with more than 50 billion birds produced annually for consumption. Specialized breeds such as broilers and laying hens have been developed for meat and egg production, respectively. A hen bred for laying can produce over 300 eggs per year. Chickens are social animals with complex vocalizations and behaviors, and feature in folklore, religion, and literature across many societies. Their economic importance makes them a central component of global animal husbandry.

Etymology

The word chicken comes from an old English word, cicen. Even though it has German roots, we aren’t exactly sure where the word originated from. It might have come from trying to copy the sound a chicken makes.

Nomenclature

Here are some common words for chickens:

• Biddy: a chicken or a newly hatched chicken
• Capon: a male chicken that has been neutered
• Chick: a young chicken
• Chook: a chicken (used in Australia and New Zealand)
• Cock: a fertile adult male chicken
• Cockerel: a young male chicken
• Hen: an adult female chicken
• Pullet: a young female chicken less than a year old
• Rooster: a fertile adult male chicken, especially in North America
• Yardbird: a chicken (used in some parts of the southern United States)

The word "chicken" can also mean a young chick. This is how the word was used long ago, like in a play by William Shakespeare called Macbeth. It is still seen in place names such as the Hen and Chicken Islands. Sometimes chickens are called "common fowl" or "domestic fowl".

Description

Chickens are fairly large birds that are active during the day. They have round bodies, unfeathered legs, and short wings. Unlike their wild relatives, chickens cannot fly far because they are too heavy. Their size and colors differ a lot between types. Baby chicks weigh about 37 g when they hatch, but modern chickens grow much faster than older types.

Both male and female chickens have a special fleshy crest on their heads called a comb and flaps of skin under their beaks called wattles. These are usually bigger on males. Some chickens even have extra feathers under their faces, giving them a bearded look. Chickens eat many things, searching the ground for seeds, insects, and small animals. They can live between 5 to 10 years, depending on their type. Chickens live in groups and take turns getting food and nesting spots. They make different sounds to talk to each other and warn of danger.

Reproduction and life-cycle

Some roosters may dance around a hen to show interest. If she responds, the rooster may climb on top and mate with her. After mating, the male shows a special display with his tail.

Sperm transfer happens through a special contact between the male and female called the 'cloacal kiss'. Like all birds, chickens control their reproduction through a special system in their brains. Hormones help start and keep the changes for reproduction.

Hens often try to lay eggs in nests that already have eggs and may move eggs from other nests into their own. A flock usually uses just a few favorite spots for nesting. In natural conditions, most hens lay eggs until their clutch is complete, then they sit on the nest to keep the eggs warm until they hatch. This is called going broody.

Chickens from high-altitude areas in Tibet have special adaptations in their eggs that help them hatch better in low oxygen areas. When placed in such environments, the embryos in these eggs produce more hemoglobin, which helps them get oxygen more easily.

Fertile chicken eggs hatch after about 21 days. The chick uses a small bump called an egg tooth to break out of the shell. After hatching, the hen stays close to her chicks for about two days. During this time, the chicks feed by absorbing an internal yolk sac. The hen protects her chicks, keeps them warm, and leads them to food and water. The chicks quickly learn to follow the hen. She continues to care for them until they are several weeks old.

Origin

Water or ground-dwelling fowl similar to modern partridges, in the Galliformes, the order of bird that chickens belong to, survived the Cretaceous–Paleogene extinction event. Chickens are descended primarily from the red junglefowl (Gallus gallus) and are scientifically classified as the same species. Domesticated chickens can freely interbreed with populations of red junglefowl.

Further information: Domestication

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. Humans took advantage of the red junglefowl's ability to reproduce prolifically when exposed to a surge in its food supply.

Genomic studies estimated that the chicken was domesticated 8,000 years ago in Southeast Asia and spread to China and India 2,000 to 3,000 years later. Archaeological evidence appeared to support domestic chickens in Southeast Asia well before 6000 BC, China by 6000 BC and India by 2000 BC. An early study proposed that a single domestication event of the red junglefowl in present-day Thailand gave rise to the modern chicken. A 2020 Nature study suggests that all domestic chickens originate from a single domestication event of red junglefowl (Gallus gallus) whose present-day distribution is predominantly in Southeast Asia. These domesticated chickens spread across Southeast and South Asia where they interbred with local wild species of junglefowl, forming genetically and geographically distinct groups.

Re-examination of bones from over 600 sites, and dating of those from 23 sites, identified the earliest probable chicken bones as from central Thailand, at Ban Non Wat, some 3,250 years ago. The paleo-anatomist Joris Peters and the bioarchaeologist Greger Larson state that this coincided with the growing of rice, and propose that wild jungle fowl were attracted to eat the rice seeds, nested nearby, and became domesticated. Skeletons of birds in the Gallus genus were used as grave goods at the site, confirming domestication.

Dispersal

Austronesia

The word for chickens in ancient languages shows that they were raised by people from very old times. Chickens traveled with people across many islands and places, starting from around 3000 BC from Taiwan. They may have reached South America long ago, but this is not certain.

Africa

Chickens came to Egypt from the Middle East around 1400 BC and were raised there by about 300 BC. They spread to Africa through different paths, including trade routes and travel.

Americas

Some researchers think chickens might have been in the Americas before European contact, based on special blue-egged chickens found there and in Asia. However, there is not enough information to be sure, and more study is needed.

Eurasia

Finding the age of chicken bones is hard because they are small and break easily. There are clues from old writings and art that show chickens were common in places like Central Asia by the 4th century BC. Chickens were in the Middle East by before 2000 BC and spread around the Mediterranean by people from Syria. By the time of the Roman Empire, raising chickens became more common.

Diseases

Main article: Poultry disease

Chickens can get sick from tiny creatures called parasites like mites. They can also catch diseases caused by tiny living things called pathogens, such as bacteria and viruses. One parasite, Dermanyssus gallinae, feeds on the chicken’s blood, which can bother the chicken and lower the number of eggs it lays. This parasite can also spread some bacterial diseases like salmonellosis and spirochaetosis. There are also viral diseases, such as avian influenza.

Use by humans

Main article: Poultry farming

Chickens are very common animals that people keep on farms. There are over 23 billion chickens in the world, and more than 50 billion are raised each year for their meat and eggs. In the United States, about 8 billion chickens are raised for meat, and over 300 million are raised to lay eggs. Most chickens are raised on special farms called factory farms, where they live in controlled environments. Some people prefer free-range farming, where chickens can move around more freely.

Keeping chickens as pets has become popular, especially in cities and towns. Many people enjoy their company and find them intelligent and entertaining. Chickens can be friendly, eating from your hand and even following you around. Some breeds are especially good for children because they are gentle and calm.

Main article: Cockfight

Chickens have also been used in science to study how animals develop. Their eggs make it easy for scientists to observe growth and make discoveries about body parts.

Main article: Cultural references to chickens

Chickens appear in many stories, traditions, and artworks around the world. They are important in some religions and cultures, and they have inspired many pieces of art and literature. One famous question is, "Why did the chicken cross the road?" Chickens have also been featured in movies and nursery rhymes, like "Cock a Doodle Doo."

See also: Chicken as biological research model and Shell-less chick embryo culture