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 neither direct descendants nor especially close relatives. They are, however, frequently used as models for theropod research due to their anatomical similarities to non-avian theropods.
Description
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.
Growth
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.
Sexual Dimorphism
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.
Habitat
Preferred Habitat
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.
Natural range
This bird originated in South Asia, living in the warm woodlands, and was domesticated between 6000 and 3000 BCE. Countries where it lived in the wild include India, Bangladesh, Myanmar, Laos, Vietnam, Cambodia, Thailand, Malaysia, Singapore, and Indonesia. Red junglefowl are still found in the wild in these countries, as are domestic chickens both in captivity and having gotten into the wild. The majority of domestic chickens live on farms.
Introduced
Bred for a variety of resources around the world, the chicken has become the world’s most populous bird, with over 19 billion worldwide as of 2011. By extent, due to being the most common bird, the chicken is also the most common extant dinosaur in the world (and possibly the most common of all time). The only continent lacking chickens is Antarctica, since that is the only continent without a permanent human settlement. Even the world’s most isolated islands such as the Hawaiian Archipelago have chickens, which were brought there by the peoples of the Pacific throughout history.
Each year, more than fifty billion chickens are reared from the egg each year. Most of these are bred for food, and a large percentage do not survive infancy in modern factory farms. Due to the prevalence of this intensive indoor farming method, the vast majority of the world’s chickens live in cages, and virtually none survive (either dying due to neglect during youth or being slaughtered for their meat as adults). Live chickens are sometimes sold, but these are mainly at local markets, for example one seen in 1993 in San José, Costa Rica. Those chickens which live on locally-owned independent farms generally have better lives: examples of these include the Sorkin family farm in Arkansas and the Peréz farm in Texas, both located in the United States. Compared to other domestic animals, there are fairly few feral chickens.
Behavior and Ecology
Activity Patterns
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.
Social Behavior
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.
Reproduction
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.
Communication
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.
Ecological Interactions
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.
Cultural Significance
Symbolism
Since they are very common domestic animals and are familiar to nearly everyone, chickens are frequently depicted in art and cultural references. They are commonly used as characters in children’s literature, fables, and fairy tales; they are perhaps the most commonly featured bird in both European and Asian cultures. Due to the expansion of European empires during the colonial era, chickens are widely known around the world.
What exactly the chicken symbolizes in culture is highly variable. The word “chicken” in English is often synonymous with cowardice, and is used as an insult toward a person who is fearful; this is probably due to the chicken’s seemingly nervous behavior as it makes quick, precise movements. Its main defense is to flee, which is actually a fairly reliable defense mechanism for a small prey animal. The chicken is also stereotyped as unintelligent in Western society as it lives on farms where it is exploited for food, but it is probable that most chickens are either unaware of their ultimate fate or are unable to escape it. On the other hand, the term “mother hen” is often used to describe devoted parents; naturally this is most often applied to mothers, but on rare occasions is heard describing fathers as well (sometimes as an insult toward the individual’s masculinity, as used by Vic Hoskins against Owen Grady). The motherly aspects of hens are complemented by the representation of the rooster as a strong patriarch, though it is only infrequently used as a comparison to humans. More generally, chickens and the rooster’s distinctive crow are seen in the United States as fundamentally recognizable symbols of farm life and agriculture.
In Captivity
Many people choose to keep chickens in domesticity for food or as pets. Ancient historians described this creature as laying an egg every day, and while this is perhaps an exaggeration, chickens do lay fully-formed and nutritious postovulatory eggs with reliable regularity. This makes them an appealing domestic animal, providing a versatile source of protein. Keeping a population of chickens makes for a large supply of eggs that can be turned into a local industry by a dedicated farming family.
Chickens are outdoor animals by nature and are best kept in moderately-sized enclosures where they have access to soil and protection from wild animals. They may be preyed upon by birds of prey as well as carnivorous mammals, so their enclosures must be secure from the sides as well as above. Wire fencing will hold off most predators. Being able to reach the soil will ensure that the chickens can access the foods that they prefer, such as seeds, worms, and insects; this can also benefit the farm by eliminating pests and weeds. In fact, many modern small farms make use of a “chicken tractor,” a mobile chicken enclosure that can be moved from one region of agricultural plot to another, allowing the chickens to feed on all the undesirable plants and animals living in the soil.
During the night, the chickens will need a place to roost, and so it is common to build them a coop where they can sleep in safety. These buildings should contain nests where the chickens will rest and lay their eggs. Unfertilized postovulatory eggs can be collected and used for food, while fertilized eggs can either be collected and hand-reared or hatched by the parent and raised among the flock. It is best to keep only one rooster for an average-sized flock, since rival males may fight and injure one another. So long as peace is maintained, the chicken flock will become highly productive and a welcome addition to the farm. These are reasonably intelligent birds, capable of bonding to their caretakers and demonstrating affection for humans they are close with. This makes the chicken an ideal domestic fowl.
Science
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. However, the chicken is not a direct descendant of any non-avian theropod, as birds split off from the rest of the theropod lineage many millions of years before the Cretaceous-Paleogene mass extinction event. The chicken is not an especially close relative of non-avian theropods either, contrary to common belief; all modern birds are fairly equal in terms of evolutionary proximity. The chicken is a common and easily accessible bird, which along with its anatomical similarities is the primary reason it is associated with non-avian theropods.
Theropod research has branched into the field of genetics in the modern day, and chickens are often used to study which genes control dinosaurian physiological traits. Modern birds lack teeth, snouts, bony tails, and clawed fingers, but through genetic manipulation they can be bred to have these features. This provides geneticists and paleontologists with insight into avian evolution that benefits our understanding of modern birds as well as their ancestors, a particularly important field of study now that dinosaurian de-extinction is mainstream. Some paleontologists, such as Dr. Jack Horner, have advocated using genetic engineering to breed chickens which closely resemble non-avian theropods, but it is unclear what use this would be other than being aesthetically interesting.
Politics
Like all domesticated animals, chickens are central in the animal rights movement, particularly due to the cruel practice of factory farming (as discussed below). Chickens in factory farms are a major source of zoonotic disease, which can become an international issue if left unchecked since the chicken trade is global in scale. These commonly-eaten birds are a massive force in the global economy, and are a major export of numerous countries. Some smaller communities rely on production of chicken eggs and meat to support themselves. Since the chicken is a comparatively easy animal to keep in captivity, it comes as a boon to impoverished communities in the global south that have been devastated by imperialism.
While it is not generally debated in government, the eating of chicken eggs is a hot topic of debate in vegetarian and vegan culture. Since the unfertilized eggs of chickens are of no further use to the birds themselves, and are regularly produced without human intervention, some vegetarians and vegans believe that eating the eggs is permissible and does no harm to the animals. Others argue that since they are an animal product, they should not be consumed as a matter of principle, and that eating them could inadvertently lead to exploitation.
Resources
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.
Chicken meat is used to make a wide variety of dishes around the globe. For example, a pan-roasted chicken meal was a menu option at Winston’s Steakhouse in Jurassic World.
Chickens have been renowned since ancient times for their fecundity, and have been raised for their eggs for thousands of years. 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.
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.
Safety
Chickens are generally not dangerous, but their beaks and claws are sharp and can cause harm. Roosters in particular have spurs on their feet which they use to fight rival males and predators, and will use these to attack a human they feel threatened by. To avoid provoking a rooster, you should not stare or move directly at it, nor should you make obvious effort to creep around it. Instead, move calmly and confidently around the area without making any sudden movements toward the rooster. Until your birds are fully comfortable around you, it may be best to wear tall rubber boots, gloves, jeans, and long sleeves in case the rooster attacks.
If a rooster should become aggressive, it will probably batter you with its wings while clawing and pecking. Protect your face and any exposed skin as best you can. If you are carrying food, try distracting the rooster with it; while feeding wild animals is inadvisable, training your domestic chickens to associate you with food is actually ideal, as it will help them to trust you. This can reduce the likelihood of a rooster attack in the future. When moving in and out of the enclosure (for example, to collect eggs), carrying something to use as a shield will work in your favor as you can block the rooster’s flapping and clawing attacks. You can try briefly pinning it down, though do take care not to be excessively forceful, and releasing it once it appears ready to flee rather than fight.
Free-range chickens may unintentionally encounter people who are not their owners, which can startle them. If you are walking through an area where you know free-range chickens (and particularly roosters) to live, carry a walking stick, bear spray, or other items to fend off a startled or territorial rooster. Overall, your goal when interacting with roosters and their flocks should be to appear uninteresting: a rooster will only attack something it thinks is a threat to its flock or its status in the pecking order. So long as you fit neither of those criteria, a rooster will probably not attack you.
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.
