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The Body Systems of Birds
Todd A. Culver 

There are many physical features that are shared by birds and mammals. For instance, both groups of animals have backbones, are warm-blooded, and have four limbs. However, the body systems of birds are adapted to flight.

Bones and Muscles
Over time, the skeleton of the bird has developed into an airy, lightweight, yet strong, frame. Since the earliest birds, many body parts, such as teeth, have disappeared - lightening the frame. Other parts, such as the vertebrae of the back, have become fused (joined) - giving the frame more strength.

While a bird's many neck vertebrae are shaped so that the neck is extremely flexible, most of the rest of the skeleton has become stiffened. Birds have a large breastbone, or sternum, that protects the internal organs and provides strong support for the attached muscles that power flight. The heaviest, strongest bone is the coracoid. Together with the ribs, it holds the sternum and the rigid backbone apart when the flight muscles contract.

The largest muscles in a bird are the pectoral (breast) muscles. These muscles may account for as much as one fifth of a bird's entire weight. They are attached to the long bone of the wing. When they contract during flight, the wings are pulled down. Smaller muscles, called the supracoracoideus, contract during flight and the wings are pulled up.

Digestive System
Since birds have no teeth, the digestive system must grind up food so that the energy stored in it can be used. A hollow digestive tube extends from the mouth through the entire length of the body to an external opening called the cloaca. The tube is divided into the throat (pharynx), esophagus, stomach, small intestine, and large intestine.

Food passes from the mouth through the pharynx into the esophagus. Some birds, such as pigeons, doves, and hawks, have a portion of their esophagus enlarged to form a storage pouch called a crop. The crop allows birds to feed quickly and digest its meal later in safety.

Nearly all birds have a stomach made up of two parts. The first part of the stomach, called the proventriculus, secretes strong digestive juices, which in some birds are strong enough to digest bones! The second part of the stomach, called the gizzard, has strong muscular walls that act like teeth to grind and pulverize foods. In addition to the muscular walls, birds often swallow small pebbles and grit that get trapped in the gizzard and aid in the grinding process.

From the stomach, food passes into the small intestine where the nutritious end products of digestion are absorbed into the bloodstream. The remaining indigestible matter travels into the large intestine where it is stored until it is eliminated from the body through the cloaca.

Metabolic System
A bird's metabolic system guides all the chemical changes that provide energy for vital processes and activities such as new cell growth and maintenance of body temperature. Birds, which have a higher body temperature, a faster heart rate, and a greater need for oxygen than mammals, must eat a great deal of food to get the necessary energy to fuel the body functions. Small birds have a relatively higher metabolic rate, and therefore higher energy needs, than large birds. An ostrich can go several days without food, however, a small hummingbird needs so much energy that it must feed almost constantly during the day.

Birds are endothermal, or warm-blooded. That means they have an internal furnace, fueled by food, that generates heat and allows them to keep their bodies at a constant temperature, even though the temperature of their environment changes. Birds are also able to regulate their body temperature by conserving or losing heat through a variety of ways - feathers help retain heat, while panting helps get rid of heat.

Circulatory System
A bird's circulatory system consists of a four-chambered heart and blood vessels. With each beat, or stroke, of the heart, a large volume of blood is carried throughout the bird's body by vessels called arteries. Blood is then returned to the heart by vessels called veins. Birds are nature's best athletes. With their powerful hearts, they can keep up extraordinary levels of physical exertion for long periods. A ruby-throated hummingbird's heart is about 3 percent of its total body weight, but it is strong enough to supply fuel and oxygen to the flight muscles for a nonstop flight across the Gulf of Mexico.

The high metabolism of birds requires rapid circulation of the blood because waste products build up quickly in the cells and must be removed before they reach a toxic level. Typically, small birds have a higher metabolism than large birds and therefore have a faster heart rate.

Respiratory System
The high metabolism and athletic life-style of birds require a great deal of oxygen. Four organs work together to carry oxygen to the cells: nostrils, trachea, lungs, and air sacs. With each breath, air moves through the nostrils, down the trachea and into the lungs and air sacs. From the lungs, oxygen passes into the bloodstream and then to the body cells. The air that passes into the air sacs cools the internal organs and helps maintain body temperature. Nearly all of the air in the lungs is replaced with each breath. When flying, birds require ten to twenty times more oxygen than at rest. To supply the extra oxygen, birds increase their breathing rates.

Nervous System
The nervous system of a bird consists of the brain, sense organs, and nerves. Nerves carry messages from sense organs, such as the eyes, to the brain. Nerves also carry messages from the brain to the muscles. The term "birdbrain" may need to be forgotten. Researchers have found that although birds lack a cerebral cortex - the brain structure associated with intelligence in mammals - they can still pass messages between higher and lower brain regions. This functionality helps explain how birds are capable of complex behaviors.

As a group, birds have the best vision of all animals. Their large eyes, which sometimes weigh more than their brain, provide keen sight and excellent color perception. Birds that are most active at night, or nocturnal, also have well-developed sight. Range of vision depends on whether the eyes are on the sides of the head or on the front of the head.

Hearing is also well developed in birds, with some night birds having especially acute hearing. Only a few birds, such as the kiwi, have a highly developed sense of smell. The kiwi is nearly blind and relies on its sense of smell to find food.

Reproductive System
The reproductive system includes the sex organs and a series of tubes, or ducts, which act as passages for the sex cells produced within the organs. The male sex organs, called the testes, produce sperm; the female sex organs, called the ovaries, produce eggs. When birds mate, sperm from the testes pass from the male to the female. The sperm fertilize the eggs. After an egg is fertilized, it begins to travel down a narrow tube called the oviduct toward the external opening, or cloaca. On the way down the oviduct, it first receives a coating of the protein called albumen (the egg white). Further down the oviduct, the albumen is surrounded by a shell composed of calcium. As the egg nears the end of the oviduct and before it is laid, it also may receive various colored pigments.
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