Chicken

• 1. Introduction

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  The chicken began as a wild bird scratching for bamboo seeds in the forests of Southeast Asia. Today the global chicken population exceeds 26.5 billion, and more than 50 billion birds are produced each year for people to eat. That makes Gallus gallus domesticus one of the most common and widespread domesticated animals on Earth. It descends from the red junglefowl, a bird so genetically close that the two are classified as the same species. How did a forest bird become a creature humans count by the tens of billions? Why was it first prized not for its meat or its eggs but for its willingness to fight? And what can a cracked-open egg teach a scientist about the way a body grows its limbs? The answers run from rice paddies in central Thailand to the pages of a Nobel Prize-winning novel.

• 2. A Bird Of Many Names

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  The word chicken comes from Old English cicen, pronounced much as it is today. Its roots are Germanic, but exactly where it came from is unclear; it may ultimately imitate the sound the bird makes. Historically, chicken meant a chick rather than the species. William Shakespeare used it that way in Macbeth, where Macduff mourns the death of "all my pretty chickens and their dam". The older sense survives in placenames like the Hen and Chicken Islands. The English language carries a small flock of words for these birds. A capon is a castrated male; a cockerel is a young male; a pullet is a young female under a year old. In the poultry industry a pullet is a sexually immature bird less than 22 weeks of age. The word rooster, a fertile adult male especially in North America, originated in the 18th century. It may have been a euphemism to avoid the sexual connotation of the word cock. In trade and scientific contexts, the species is still often called common fowl or domestic fowl.

• 3. Anatomy And Instinct

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  Newly hatched chicks weigh the same whether modern or heritage, about 37 grams. From there the paths diverge sharply. By day 35 a Ross 708 broiler may weigh 1.8 kilograms, while a heritage chick of the same age reaches only 1.05 kilograms. Adult chickens carry a fleshy crest called a comb on their heads and hanging flaps of skin called wattles under their beaks, both more prominent in males. Some breeds have a mutation that adds extra feathering under the face, giving the appearance of a beard. Wild junglefowl can fly, but domestic chickens carry flight muscles too heavy to lift them more than a short distance. In the wild these omnivores scratch the soil for seeds and insects, and they will take prey as large as lizards, small snakes, and young mice. A chicken may live 5-10 years depending on the breed, though the world's oldest known chicken reached 16. Their social lives are intricate. In 1921 Thorleif Schjelderup-Ebbe described dominance among female chickens as the "pecking order", in which dominant birds take priority for food and nest sites. Males settle conflicts differently, leaping and using their claws. A flock can mob and kill a weak or inexperienced predator, such as a young fox. Their calls carry meaning too: a rooster's crow signals territory, a hen clucks after laying or to gather her chicks, and different warning calls tell the flock whether a predator approaches from the air or the ground.

• 4. Courtship And The Nest

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  Some roosters open courtship with a circle dance, stepping around a hen and lowering the wing closest to her. The display triggers a response in the hen, and if she answers his call he may mount and mate. The sequence includes a waltzing display by the male; if the female is unreceptive she runs off, and if she crouches he treads on her back with both feet. Sperm transfer happens through cloacal contact, an action called the "cloacal kiss". As in all birds, reproduction runs through a neuroendocrine system, the Gonadotropin-Releasing Hormone-I neurons in the hypothalamus. Hens prefer shared real estate, often laying in nests that already hold eggs and sometimes moving eggs from neighbouring nests into their own. A flock therefore concentrates on a few preferred locations rather than one nest per bird. When a clutch is complete a hen goes broody, sitting tight and pecking defensively if disturbed, rarely leaving until the eggs hatch. Chickens from the high-altitude region of Tibet carry a striking adaptation. In low-oxygen conditions their embryos express far more hemoglobin than other populations, and that hemoglobin binds oxygen more readily, raising the hatching rate. Fertile eggs hatch after about 21 days, the chick breaking the shell with its egg tooth. The hen stays on the nest about two days after the first chick hatches while the newborns feed by absorbing the internal yolk sac. The chicks imprint on her and follow her continually as she leads them to food and water and broods them for warmth.

• 5. Tracing The Origin

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  Water or ground-dwelling fowl resembling modern partridges survived the Cretaceous-Paleogene extinction that killed all tree-dwelling birds and their dinosaur relatives. From that surviving order, the Galliformes, the chicken eventually emerged, descended primarily from the red junglefowl. The two interbreed freely, and chickens share between 71 and 79 percent of their genome with red junglefowl. The domestic bird later hybridised with other species: a gene for yellow skin, for instance, came from the grey junglefowl, Gallus sonneratii. The story of when and where this happened was long contested. Genomic studies estimate domestication around 8,000 years ago in Southeast Asia, with spread to China and India 2,000 to 3,000 years later. A 2020 Nature study that sequenced 863 chickens worldwide pointed to a single domestication event of red junglefowl whose present-day range is mostly Southeast Asian. The most popular commercial breed reflects this tangled past: the White Leghorn carries a mosaic of divergent ancestries from different subspecies of red junglefowl. A close look at the bones sharpened the picture. Re-examination of remains from over 600 sites, with dating at 23 of them, identified the earliest probable chicken bones at Ban Non Wat in central Thailand, some 3,250 years ago. The paleo-anatomist Joris Peters and the bioarchaeologist Greger Larson tie this to the growing of rice, proposing that wild jungle fowl came to eat the rice seeds, nested nearby, and were domesticated. Bird skeletons in the Gallus genus served as grave goods at the site, confirming the relationship.

• 6. Carried Across The World

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  A word for the domestic chicken, reconstructed as *manuk, belongs to the Proto-Austronesian language, marking the bird as an ancient companion of the Austronesian peoples. Together with dogs and pigs, chickens travelled the full range of the prehistoric Austronesian sea migrations, reaching Island Southeast Asia, Micronesia, Island Melanesia, Polynesia, and Madagascar, starting from at least 3000 BC from Taiwan. Some researchers suggest Polynesian seafarers may have introduced them to South America before Columbus, though this is disputed. The route into Africa ran through older empires. Chickens reached Egypt by way of the Middle East for cockfighting about 1400 BC and were widely bred there around 300 BC. Three possible paths carried them deeper into the continent around the early first millennium AD: through the Egyptian Nile Valley, through East African Roman-Greek or Indian trade, or from Carthage and the Berbers across the Sahara. The earliest known remains span Mali, Nubia, the East Coast, and South Africa, dating to the middle of the first millennium AD. Europe and the Middle East offer their own milestones. Chicken remains in Syria go back a little earlier than 2000 BC, and Phoenicians spread the birds along the Mediterranean coasts as far as Iberia. The first pictures of chickens in Europe appear on Corinthian pottery of the 7th century BC. Genetic sequencing of European chicken bones shows that in the High Middle Ages the birds became less aggressive and began laying eggs earlier in the breeding season.

• 7. The Industrial Bird

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  More than 8 billion chickens are slaughtered each year for meat in the United States alone, and more than 300 million are reared there for eggs. The vast majority of poultry is raised in factory farms; according to the Worldwatch Institute, this method produces 74 percent of the world's poultry meat and 68 percent of its eggs. Birds raised for meat are called broilers, and broiler breeds typically reach slaughter size in less than six weeks. The friction between intensive farming and free-range farming has fed long-running debates over ethical consumerism. Critics argue intensive systems harm the environment and treat sentient animals inhumanely, while advocates say efficient systems save land and food through higher productivity. Egg production tells a parallel story of pushed biology. Layer hens of some breeds produce over 300 eggs a year, and the highest authenticated rate is 371 eggs in 364 days. In the UK alone people consume more than 34 million eggs per day. After 12 months of laying a commercial hen's output falls until the flock is no longer viable. Hens from battery cage systems may be infirm or have lost many feathers, their life expectancy cut from around seven years to less than two. In the UK and Europe they are then slaughtered for processed foods or sold as soup hens. Elsewhere, flocks are sometimes force-moulted instead, with food and sometimes water withdrawn for 7-14 days, or long enough to cause a body weight loss of 25 to 35 percent. The shock makes the hen lose her feathers and re-invigorates egg production. In 2003, more than 75 percent of all flocks in the US were moulted.

• 8. Pets, Fighters, And Science

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  Keeping chickens as pets grew popular in the 2000s among urban and suburban residents. Owners often name their birds and treat them like cats or dogs, finding that chickens eat from the hand, jump onto a lap, follow their handlers, and show affection. Docile breeds such as silkies and many bantam varieties are often recommended for children with disabilities. Long before chickens were companions, they were combatants. A cockfight pits two cocks in a ring called a cockpit, and the practice appears in the Indus Valley civilisation from 2500 to 2100 BC. In domestication, chickens were apparently kept first for cockfighting and only later used for food, a reversal of how most people imagine the bird's purpose. The same animal became a workhorse of the laboratory. Embryologists open a fertilized egg, observe the developing embryo, then close the shell and study the effects later, a technique behind discoveries in limb development such as the apical ectodermal ridge and the zone of polarizing activity. The chicken was also the first bird species to have its genome sequenced, measuring 1.21 gigabases against the human genome's 3.2, with 19,119 protein-coding genes. The bird even carries echoes of its deep past. In 2006 scientists switched on a recessive gene called talpid2, and the embryo's jaws began forming teeth like those of ancient bird fossils. The chicken also lives in story and belief, from the divination practice of alectryomancy to Gabriel García Márquez's 1967 novel One Hundred Years of Solitude, where cockfighting is outlawed in Macondo after a Buendia patriarch kills his rival and is haunted by the man's ghost.