The chicken (Gallus gallus domesticus) is a domesticated subspecies of red junglefowl, which was first raised by humans between 6000 and 3000 BCE. It is one of the most common and widespread domestic animals, raised in captivity by humans as a source of eggs, meat, sometimes feathers, and occasionally for companionship. Originating in South Asia, this animal is now more numerous than any other species of bird in the world.
Chickens belong to the order Galliformes, one of the most primitive groups of birds. They are closely related to the small ground-dwelling birds which survived the Cretaceous-Paleogene mass extinction event, which caused the extinction of the non-avian dinosaurs. As a result, chickens are usually (and incorrectly) said to be the descendants of dinosaurs such as Tyrannosaurus rex, though in reality they are simply close relatives. They are, however, among the more primitive of birds, and are frequently used as models for theropod research.
Other subspecies of red junglefowl include:
- Gallus gallus gallus, found throughout Southeast Asia
- Gallus gallus bankiva, found in Java
- Gallus gallus jabouillei, found in Vietnam
- Gallus gallus murghi, found in India and Bangladesh
- Gallus gallus spadecius, found in Burma
The chicken is not extremely different from the wild red junglefowl, though many breeds exhibit unique appearances and can be easily told apart. Most chickens in captivity appear plump, both due to their coat of feathers and the provision of food by their keepers. They are usually larger but duller in color than their wild counterparts. A typical size is around 70 centimeters (28 inches) is common in males, where the tail may constitute half this length.
A chicken has a comparatively small head and short, hooked beak; it is an omnivore, and its anatomy reflects this. Its beak is able to selectively pluck both plant and animal food from the ground, and its eyes give it a three-hundred-degree range of vision. Its eyes can move independently of one another. Chickens have excellent eyesight, due in part to having an unusual state of matter in their eyes called disordered hyperuniformity. Their color vision exceeds that of the human. On the top of its head, a chicken may have a fleshy structure called a coxcomb or simply the comb, which is typically red and resembles a waved or pointed fin. There may also be wattles hanging down from either side of the face; the coxcomb and wattles together are called caruncles. Some chickens have a beard of feathers in addition to or in place of caruncles.
While the neck of the chicken can be longer than it appears, it is often concealed with a layer of fat and feathers. Its body is bulkier than those of flying birds; the chicken may be capable of brief flight if permitted to exercise, but even in the wild, it usually does not fly any more than necessary to clear small obstacles or escape danger. Its wings are shorter and rounder than flying birds’ wings, and its tail feathers are usually short; in the male, the tail feathers may be longer. In the ancestral red junglefowl, there are fourteen tail feathers in total.
Its legs, much like the neck, are longer than they appear due to being concealed by fat and feathers. The lower legs and feet are usually left exposed in most breeds and are scaly. The feet are anisodactylous, meaning three toes face forward and one faces backward. The toes bear small curved claws which help the chicken gain purchase on the ground.
Coloration in chickens is highly variable. Most of the body is covered in feathers, which may be white, brown, red, orange, yellow, black, or iridescent green. The caruncles are usually red, but may be pink or gray. Generally the featherless, scaly parts of the legs and feet are yellow, as is the beak; the scaly lower legs may also be other colors, usually black, orange, white, or gray.
There are hundreds of chicken breeds, each of which has distinct characteristics.
Hatchling chickens are often called chicks. When they hatch, they possess a layer of down feathers which give them a fluffy appearance. At this stage, they are often duller in color than the adults, frequently appearing with subdued brown and white colors. Some breeds have yellow down at this age.
When less than a year old, females are called pullets, and males are called cockerels. Maturity is reached fairly quickly, with females able to lay eggs within a few months. At the adult stage, which is usually reached after one year, female chickens are called hens and males are called cocks or roosters (“cock” is the preferred term in the United Kingdom and Ireland, while “rooster” is the preferred term in the United States of America, Canada, Australia, and New Zealand).
Hens gradually produce fewer and fewer eggs after passing the one-year mark, and by the time they are five to seven years old, they may stop laying eggs altogether. This is called henopause. The overall lifespan of a domestic chicken is eight to fifteen years.
Depending on the breed, sexual dimorphism in the chicken can take one of several forms. Roosters, in most breeds, are brighter in color than hens. The rooster has longer, pointed feathers on his neck (called hackles) and back (called the saddle), though in some breeds these are less prominent. The rooster of many breeds has a much more prominent set of caruncles, though the height of the comb and length of the wattles may vary from one breed to another.
Tail feathers are generally more prominent in roosters. This is the ancestral state of the red junglefowl, in which the cock may have 28-centimeter (11-inch) tail feathers which are arched in shape and vibrantly colored. In addition, the male can be told apart by the presence of leg spurs, which are used in combat.
Chickens are originally native to forested environments; they are a subspecies of red junglefowl, which lives in warm, rainy regions. However, this highly adaptable bird has been acclimated to many different climate types around the world with much success. Perhaps the only environments it cannot inhabit are deserts and polar regions.
There is currently no evidence that chickens have ever been introduced to Isla Nublar.
There is currently no evidence that chickens have ever been introduced to Isla Sorna.
Due to humans breeding them for various purposes, the chicken is the most common bird (and, by extent, the most common dinosaur) in the world. As of 2011, there were over 19 billion chickens worldwide, living on every continent except for Antarctica and even existing on some of the world’s most isolated islands. More than fifty billion chickens are reared from the egg each year, with the vast majority living their entire lives in factory farms and being slaughtered for food if they survive infancy.
Behavior and Ecology
The chicken is a diurnal animal, often waking up early in the morning and feeding throughout the day. In the evening, it returns to a roosting area to sleep. It will choose a secure roost where it feels that predators cannot reach it; in the wild, this is often a raised area such as a tree, while in captivity humans will typically provide a small building called a coop. However, domestic chickens in factory farms will have more erratic activity patterns due to having no concept of day, night, or the passage of time.
Diet and Feeding Behavior
Chickens are omnivores, feeding primarily on seeds, fruits, grains, and small invertebrates. They use their beaks to pluck food off the ground, swallowing it whole. A chicken may also use the claws of its feet to scratch through the soil in search of food.
On occasion, chickens may feed on larger animals. They have been known to eat large invertebrates, lizards, snakes, and small rodents.
Like many birds, the chicken is a highly gregarious animal. Social groups (called flocks) can be quite large, and are often dominated by one or a small number of roosters, depending on the flock’s size. A distinct social hierarchy is in place, with more dominant chickens having the best access to food and nesting locations; this is where the term “pecking order” originates. If one animal is removed from the flock, the hierarchy will be disrupted until a new pecking order is established. If one is introduced to an established flock, especially if it is younger, it may be abused by the more dominant chickens.
Roosters are very territorial, and will make cries to warn away competing roosters. If confronted by a rival, they may fight using the spurs on their legs. The rooster will look after the safety of the hens, many of which are likely his mates, as well as his offspring. When he finds food, he will bring the other chickens to the food source by calling them and demonstrating the food. Mother hens will bring their offspring to food sources in the same way, encouraging them to eat.
Humans have long valued chickens for their egg production; hens may lay eggs on a daily basis even if not fertilized. These unfertilized eggs are perhaps more familiar with humans who do not raise chickens themselves. Due to their primarily existing in captivity, chickens can be bred at any time of year; in the wild, they breed during the spring and summer months.
To court a hen, the rooster will often perform a dance around the hen. This dance may involve head-bobbing, clucking, and the presentation of food. Roosters may also lower one wing while circling around their intended mate, generally the wing that is closer to the hen. If the hen is satisfied with his performance, she will crouch down to allow him to mate. However, if she is not impressed, she will run away. A rooster may mate with multiple hens throughout a breeding season.
When the hen is ready to lay her eggs, she will typically attempt to use a preexisting nest that already contains eggs rather than build a new one. These nests are constructed in safe locations and vigorously defended from predators; in some cases hens may even defend their nests from animals as large as young foxes with success. Individual chickens may prefer to use their own nests, or to gregariously share nests among the other chickens. The eggs are ovoid, often lightly speckled, and range in color from white to brown; some are naturally blue or green, and the Araucana chicken can lay purple eggs. A full clutch is about twelve, often laid over a period of a few days. The embryos only begin to develop when incubation begins, which does not occur until all the eggs have been laid. As a result, even though the eggs are laid over the course of a week or two, they will all incubate for a similar amount of time and hatch within a day or two of each other.
As the eggs incubate, the mother hens will try to keep the temperature and humidity constant. They will sit on the eggs to brood them, turning them over after the start of the incubation period. While brooding, a hen will rarely ever leave the nest for any reason.
After roughly 21 days, the chicks will begin to make sound from within the egg. The mother, hearing these noises, will gently cluck to encourage the chicks to emerge. Each chick will use its egg tooth, a specialized beak structure that exists in the embryo, to make a breathing hole in the eggshell. It will rest for some time, absorbing more nutrients from the yolk and egg membrane. Eventually, the chick will begin to break more of the eggshell around its hole, emerging completely. The mother will continue to brood for another two days or so, eventually making short trips away from the nest to seek out food and water. As the chicks grow, the mother will lead them to food so they learn how to eat. She will protect them for the first several weeks of their lives.
The best-known vocalization produced by the chicken is a clucking noise that is used for various forms of communication. Chickens may cluck to get the attention of their fellows, advising them of the locations of food and water; other forms of clucking are used by mothers to communicate with their chicks, or by roosters to court mates. Yet another clucking sound is produced by hens right after laying an egg.
Roosters have another distinctive call, a loud and shrill crowing sound that is used for territorial purposes. A rooster may hold sway over a large group of hens, warning away rivals and maintaining control using this crowing call. It is often thought by humans that this cry signals the start of morning activities, but it may actually be heard throughout the day. This crowing call is also used in response to surprising changes in the territory.
When a chicken senses danger, it will cry out loudly to warn the others. They have differing calls for when they sense a predator, using one call if the predator is approaching by land and a different call if the predator is in the air. This allows the other chickens to respond appropriately to varying predator types, even if they did not see the threat themselves.
The chicken is largely a domestic bird, and many do not have contact with the wild. Those that are permitted to roam out of doors, called free-range chickens, will prey on various small plants and animals; their food includes mainly seeds and grains, though they will eat berries and other fruits, and they may prey on invertebrates, small reptiles, and young mice. In turn, they are preyed upon by small and medium-sized carnivorous mammals and birds of prey such as hawks and owls. To defend itself, a chicken will use its beak and claws; the rooster has spurs on his legs which he can use against predators. While chickens mostly rely on staying in safe territories for survival, they are able to fight off and kill smaller predators, especially when living in larger flocks.
Chickens can be inhabited by disease-causing microorganisms, including many types of bacteria such as Salmonella, Campylobacter, and those that cause bumblefoot. They are also host to parasites, such as various intestinal worms, fungi, lice, fleas, ticks, and mites.
Relationship to Humans
For over five thousand years, humans have raised the chicken for a wide range of purposes. Throughout South and East Asia, the red junglefowl was bred for the practice of cockfighting, or pitting two roosters against one another in a violent fight. This is mostly discouraged today, as many people find animal suffering to be distasteful. From Asia the chicken spread out to Europe and Africa, where it was sometimes considered an exotic fowl. In Australasia, the chicken was a more standard domestic animal, being common throughout ancient Indonesia. From here, chickens spread across the Pacific via the seafaring Polynesians, possibly reaching South America in the distant past. They were introduced by the Polynesians even to such isolated places as Rapa Nui (Easter Island). In the modern day, chickens exist virtually everywhere humans live.
Chickens have been renowned since ancient times for their fecundity, and have been raised for their eggs for thousands of years as well. In addition, they are killed for their meat. In the United States, over eight billion chickens are killed for their meat every year, and over three hundred million are bred for their eggs. Certain hen breeds may produce more than three hundred eggs per year, but after a full year of laying, they begin to decline; they may be slaughtered at this point. In the modern day, poultry production mostly takes place in factory farms, where as many chickens as possible are placed into rows of battery cages. This maximizes efficiency of egg and meat harvesting as well as land use at the expense of the chickens’ physical and psychological health, which is detrimentally affected by the poor living conditions they exist in. Human health risks in the form of workplace hazards and zoonotic diseases are also risks of factory farming. Well over seventy percent of poultry meat is produced under these conditions, as well as almost seventy percent of eggs.
Scientists working in de-extinction and paleontology often use chickens as models for more ancient theropod dinosaurs, as these birds are fairly similar to their extinct relatives. Some smaller non-avian theropods are frequently compared to chickens; Dr. Laura Sorkin considered Compsognathus particularly chicken-like.
In the modern day, chickens have gained popularity as pets. They are intelligent and curious, and many breeds are very docile and suitable for younger children. The natural social instinct of chickens, particularly hens, makes them excellent family pets; many are known to be quite good with disabled children.
Behind the Scenes
Paleontological consultant for the Jurassic films Jack Horner has expressed interest in bioengineering a new breed of chicken to resemble a more ancient theropod. In 2006, embryologists activated the recessive gene talpid2 in chicken embryos, which caused the embryos to develop teeth as in more primitive birds. This induced atavism has been suggested as a “first step” toward the creation of chickens with more classical dinosaurian traits. Proponents of the project are optimistic about its ability to allow insight into the evolutionary biology of birds and other theropods, while its opponents express the usual ethical concerns surrounding bioengineering as well as the lack of uses for such an animal beyond evolutionary research.