Chicken is a bird that is raised for its meat and eggs. There are probably more chickens than any other single kind of bird, and they live throughout the world.
Chickens—like other birds—have feathers and wings. But chickens also have a number of special growths on their bodies that most other birds do not have. These growths include the red comb on top of the head and the red wattles that hang beneath the beak.
Chicken meat and eggs are a good source of protein. Protein is a chemical compound that is necessary for a healthy diet. Chicken meat is also low in fat. However, chicken eggs contain a large amount of a fatty substance called cholesterol. Many physicians believe that too much cholesterol in a person’s diet may contribute to heart disease (see Cholesterol).
Raising chickens for meat and eggs is a major industry in many countries, including Brazil, China, India, Indonesia, Japan, Mexico, Russia, and the United States. The world’s people eat about 130 million tons (120 million metric tons) of chicken meat each year. Hens produce about 1 trillion eggs every year.
Some people raise chickens as a hobby. They breed them for body size, and for the color and color pattern of the feathers. The birds are exhibited at fairs and livestock shows. People sometimes use feathers from the chicken’s neck and back to make flies (special hooks) for fishing. In addition, scientists may use chickens for research in medicine and other fields. Chicken eggs are used to make many vaccines, which protect human beings and animals from diseases.
The body of a chicken
Adult chickens range in weight from about 1.1 pounds (0.5 kilogram) to more than 11 pounds (5 kilograms). Feathers cover most of the body of a chicken, except for the shanks (lower legs) and feet, which have scales. The feathers help the chicken to keep warm in cold weather. Wings enable chickens to fly. However, they can fly only a few hundred feet or meters at a time. They fly mainly to escape enemies and to reach perches on which to roost at night.
The comb and wattles of chickens are fleshy structures. They are bright red because they have a rich blood supply. Earlobes grow on the sides of the head. They may be red or white, depending on the breed of chicken. The comb, wattles, and earlobes may help individual chickens recognize each other and select mates. They may also help chickens keep cool.
Chickens have claws on their feet. Males also have a bony structure called a spur on each leg. Chickens use their claws, spurs, and beak as a defense against enemies, and to dig in the soil for insects and seeds to eat. Chickens have a keen sense of sight and hearing, but their ability to taste and smell is poor compared with that of human beings. People can train chickens to perform certain simple acts in return for food.
Certain internal organs of chickens are specialized. For example, the throat contains a pouch, called a crop. The crop stores food and slowly passes it to the stomach for digestion. After the food has been mixed with digestive juices, it enters a muscular part of the stomach called the gizzard. The gizzard contains particles of sand or stone that the chicken has swallowed. These particles and the movement of the walls of the gizzard grind the food into fine pieces.
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Kinds of chickens
Chickens are grouped according to class, breed, and variety. Most classes are named for the area where the chickens were first developed. A breed consists of chickens within a class that all have a similar body type. Each variety is made up of chickens within a breed that have a certain combination of features in common. These features include the type of comb, skin color, and feather color and pattern.
There are four basic kinds of combs—the single comb; pea comb; rose comb; and cushion comb, or walnut comb. The single comb is a single blade with several points on top. The pea comb has three rows of points. Both the rose comb and the cushion comb are compact structures that do not have points. Some breeds of chickens have unusual and characteristic combs, such as the V-shaped, buttercup, and strawberry.
Feathers grow in a variety of colors and patterns. White, buff (gold), brown, red, and black are common colors. Barred feathers have bars of black separated by white or gold coloring. Mottled feathers are black with white tips. Spangled feathers are gold or white with dark tips. Some birds have a bunch of feathers on the head, called a crest. Certain crested breeds also have a muff—a thick growth of feathers on the side of the face—or a beard—a bunch of feathers under the throat.
American class chickens
have a medium-sized body, red earlobes, and white to yellow skin. All the breeds lay brown eggs, except for the Lamona and Holland, whose eggs are white. Common breeds in the American class include the Plymouth Rock, Rhode Island Red, New Hampshire, and Delaware. Most breeds of the American class were developed during the 1800’s or the early 1900’s, by crossing chickens from the Mediterranean, English, and Asiatic classes.
Asiatic class chickens
are large birds with feathers on their shanks and feet. The three Asiatic breeds—Brahmas, Cochins, and Langshans—have red earlobes and lay eggs with brown eggshells. Cochins and Brahmas have yellow skin. Langshans have white skin. The Asiatic breeds are raised mainly for show purposes.
English class chickens.
Most breeds in the English class are hearty birds that were developed for the production of eggs and meat. All breeds of the English class have red earlobes, and all except the Cornish have white skin. Only the Dorking and the Redcap lay white eggs.
Mediterranean class chickens.
Most chickens in the Mediterranean class are light in weight. They have white earlobes, and their skin is white or yellow. They are bred for egg production, and all breeds lay white eggs. Some breeds, such as the Minorca, are heavy birds and lay large eggs. The most important breeds of this class originally came from Italy and Spain. The class includes the Leghorn, Ancona, and Buttercup breeds.
Other classes.
There are many other classes that include only one or two breeds. Some of the most colorful breeds are the Houdan and Polish, which have crests on top of their heads and mottled feathers. Frizzle fowl have curled feathers. Silkies have long, slender feathers and are the only breed with black skin. The Hamburg has a rose comb with a long point. Many chickens are also bred in bantam (miniature) varieties. They are raised mainly as a hobby or for show purposes.
Chickens in the Game class are slender and stand up straighter than other breeds. They are active birds with strong legs and thighs, and are used in some parts of the world for cockfighting (see Cockfighting).
Commercially important breeds.
Certain chicken breeds have become especially important for the commercial production of meat and eggs. Generally, chickens selected to produce meat have larger bodies than do chickens selected to produce eggs. Larger birds yield more meat but tend to produce fewer eggs than do smaller birds. The single-comb white Leghorn, from the Mediterranean class, is the most important white-egg producing breed in the world. Many of the breeds used for meat production are derived from a cross between the Plymouth Rock and the Cornish breeds.
The chicken industry
There are more than 25 billion chickens in the world. Some chickens live in small flocks, supplying eggs and meat to farm families or small local markets. However, most chickens are raised on large commercial farms that are specialized to produce either eggs or meat.
Breeding and hatching.
On a commercial farm, eggs are removed from the laying house each day. The eggs are taken to other buildings, where they are allowed to develop in an incubator. An incubator is a large device in which the temperature, humidity, and air flow are carefully controlled (see Incubator). 
The embryo (unborn chick) develops rapidly inside the eggshell. It uses the egg yolk, egg white, and eggshell as its source of nourishment. After about 18 days of incubation, the eggs are placed on hatching trays in the incubator. The trays allow more room for the chicks to hatch. On the 21st day of incubation, the chicks hatch, using their beaks to break through the eggshell. Their damp bodies dry quickly, leaving them covered with short, fluffy feathers called down. Newly hatched chicks can walk, see, eat, and drink.
After hatching, chicks are sometimes sorted according to sex. Then they are vaccinated. For chicks selected to produce eggs, the tip of the beak is removed to keep the chicks from pecking each other. The chicks are then placed in specially designed boxes, and shipped to the farms where they will be raised.
Raising chicks.
Chicks that will be used for egg production are raised in wire cages or in pens with straw, wood shavings, or other absorbent material on the floor. Chickens raised for meat are kept only in pens. During the first few weeks of life, chicks require a warm environment. The temperature in a chick house may be as high as 95 °F (35 °C) for the first week. The chicks are fed mixed feeds made of ground grains, plant by-products, meat scraps, and vitamin and mineral supplements. They are given vaccines to protect them from diseases.
Producing eggs.
Only eggs that have been fertilized by mating produce chicks. However, female chickens do not need to mate to lay eggs.
Hens begin laying at about 20 weeks of age. The exact age depends on the lighting in the laying house, the breed, nutrition, and the occurrence of diseases. Farmers can control when a chicken begins laying eggs by using artificial lighting in the laying house. When lighting is used that imitates long or lengthening days, chickens will begin to lay their eggs at an earlier age. In addition, chickens lay an increased number of eggs when they are exposed to 14 to 16 hours of light per day.
Egg formation is a complicated process. It begins in an organ called the ovary, with the development of the yolk. After about nine days, the yolk is released from the ovary and enters a tube called the oviduct. The egg white, the membranes (thin layers of eggshell lining), and the shell itself are formed around the yolk as it passes through the oviduct. The addition of egg white and eggshell membranes occurs in only a few hours. But shell formation takes at least 20 hours.
Hens are generally kept for one year of egg production. The number of eggs a hen is able to lay decreases slowly during the year until the rate is too low to be profitable. Hens are then slaughtered for meat. In the United States, commercial laying hens produce an average of about 280 eggs each per year.
Eggs are gathered each day. On farms that have large flocks housed in cages, eggs are collected automatically. Eggs roll down the sloping floors of the cages onto a moving belt that carries them to processing equipment. This equipment organizes the eggs into orderly rows and then washes, rinses, and dries them.
Producing meat.
Most chicken meat comes from young, tender chickens called broilers. Broilers are often slaughtered at about 7 weeks of age, when they weigh as much as 41/2 pounds (2 kilograms). Broilers feed on a rich, high-energy diet that makes them grow fast. Some meat also comes from roasters and fowl. Roasters grow to a heavier weight than broilers, usually 6 to 8 pounds (2.7 to 3.6 kilograms), before being killed. They usually reach this weight in less than 9 weeks. Fowl consist of breeder chickens that are killed after about a year.
History
Most experts believe that chickens developed from the red junglefowl, a species of wild fowl found in Southeast Asia. People probably began to tame the chicken in prehistoric times. They bred chickens for feather color, egg color, body size, and other characteristics. Records from China show that people raised chickens there as early as 1400 B.C. As chickens were brought into Europe, they were bred to produce many varieties.
During the 1500’s, ship’s crews often took chickens on board as a source of food. Spanish explorers brought certain breeds to North America at that time. During the 1600’s, English settlers brought English class chickens to North America. Several breeds in the American class were developed during the 1800’s. At that time, the chicken industry consisted of small backyard flocks. Nearly all farms had some chickens that served as a source of meat and eggs for the farm family.
The poultry industry became important during the early 1900’s. Many improvements were made in breeding techniques. By the mid-1900’s, chickens had become the main product on many farms. Smaller farms still combine crop production and chicken or egg production. Most large poultry farms, however, do not raise crops or other animals.
