Full Title Name:  Brief Summary of the Biology and Behavior of the Domestic Chicken

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Veronica Hirsch Place of Publication:  Michigan State University College of Law Publish Year:  2003 Primary Citation:  Animal Legal and Historical Center

A brief description of the biology and behavior of the domestic chicken.

Biological Facts on the Domestic Chicken

            Archaeological evidence suggests that the bird commonly known as the chicken (Gallus domesticus) is a domesticated version of the Indian and Southeast Asian Red Jungle Fowl (Gallus gallus) which is still found in the wild today. It is thought that the bird was first tamed in China around 6000 BC, with the birds moving into India by 2000 B.C. The chicken then spread from China to Russia and from there into Europe between 750 B.C. – 42 A.D. Some scholars believe that the bird may have been domesticated first for its use in cockfighting, and only later used as a food source. The White Leghorn breed or crosses of this breed is the large white bird most commonly used in agriculture and for research, but there are over 400 different breeds of chickens.

a. Anatomy of the Chicken

            An adult male chicken is referred to as a “rooster,” adult females are called “hens,” and newly hatched birds are “chicks.” Fertilized eggs will hatch after 20-22 days and the chick will weigh just 50 grams after hatching, and between 1.5 and 3.5 kg when fully grown. The body temperature of the chicken is 103 degrees F and its respiration rate is 12-30 breaths per minute. The chicken has a four-chambered heart, with two atria and two ventricles,   which beats about 350 beats per minute, or about 6 times per second.

            The chicken has a small brain with excellent color vision and a large, well-developed hypothalamus. The bird’s skeleton is lightweight, and some of the bones, the so-called “pneumatic bones,” are hollow and act as an extension of the respiratory system. The lungs are stiff, and do not expand or contract very much as the chicken breathes. It is important to remember when holding or restraining a chicken that the breastbone of the chicken, called the “keel,” must be able to move freely or the bird might suffocate.

            Chickens do not have teeth or lips, and instead use their beaks to tear off bits of food. The tongue of the chicken is barbed to help food move backward toward the esophagus. In the esophagus is a small pouch, called a crop, where food is stored briefly before moving into the stomach where it is mixed with hydrochloric acid and pepsin. From there, the food moves into the muscular gizzard which is used to crush and grind the food, with the help of small amounts of gravel and grit that the bird has swallowed.     

            Although hens are born with two ovaries and oviducts, usually only the left one will develop and become functional. Eggs are laid by passing through the cloaca, a chamber which is also the passageway for feces via the rectum, and urine via the ureters. The outside of an egg may be contaminated by bacteria and other germs and some disease agents may pass through the shell and into the egg on occasion. Commercial poultry flocks are subject to strict disinfection, regulation and inspection procedures, and farm eggs should always be washed thoroughly and refrigerated immediately. It is also important to keep the hens healthy and their environment as clean as possible to limit the spread of disease.           

b. Behavior of Chickens

            Chickens have a rigid social structure called the “pecking order” by which every bird establishes who is dominant and who is submissive in relationship to every other bird. Dominant birds peck at submissive birds, pluck their feathers, and may chase them away or steal their food. Submissive birds will not peck back and will usually run from the dominant birds. Anytime a bird is added or subtracted from the flock, even if it is only a well-known bird that has been temporarily removed and then returned to the group, the entire flock will fight briefly to re-establish the pecking order. Flocks of greater than 15 birds can lead to excessive fighting and less productivity. Males should not be kept together as they will often fight each other and may even sexually abuse or kill the weaker birds.

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