Body of the Bird
The skeleton of all birds, except for flightless birds, such as the ostrich and rhea, is very lightweight. It contains fewer bones than the skeletons of most other vertebrates; many of these bones are either hollow or filled with tiny air pockets instead of bone marrow.
The skull is essentially like that of a reptile, and differs in many respects from both fish and mammal skulls. The bird's skull is lengthened in the front into two mandibles (jaws) that form part of the bill, or beak. Each mandible can be moved independently of the other and of the rest of the skull. The mandibles are covered with a tough, horny sheath that is similar to skin. The bill is used not only in securing food, but also as a tool for building nests and for smoothing the feathers. The upper mandible contains two slits that function as nostrils.
The backbone is very rigid, with most of the vertebrae being fused (unjointed). Because of this rigidity, the backbone provides the strong support of the back and wings needed during flight. It also allows the bird to maintain an upright posture while standing. The backbone ends in a structure called the pygostyle, or tailbone, which supports the tail feathers.
The sternum, or breastbone, has a flat outer surface, called the keel, that provides a large surface area for the attachment of the wing muscles. Three bones, the scapula, coracoid, and furcula (commonly called the "wishbone"), form the pectoral, or shoulder, girdle. This structure, unique to birds, prevents the ribs and sternum from collapsing due to the powerful contractions of the breast and wing muscles during flight.
The skeletal structure of a bird's wings is essentially the same as that of the forelimbs of other vertebrates. It consists of an upper arm bone (the humerus), two lower arm bones (the radius and ulna), wrist and palm bones (the carpals and metacarpals), and phalanges, or fingers. There are three fingers, but the index finger is the only one that is well developed. The primary flight feathers are attached here. The alula, or false wing, is an area on the thumb that consists of four small, stiff feathers. These feathers control the front edge of the wing during flight. Birds have very little flexibility in their wrists and hands because they have fewer wrist and palm bones than other vertebrates. The bones of the upper and lower arm provide areas of attachment for some of the secondary flight feathers.
In general, the bones of the lower leg and ankle of birds are proportionately smaller and have less flexibility than those of mammals. Most mammals have seven bones in the ankle, but birds have only two—the tibiotarsus and the tarsometatarsus.The canvasback has webbed feet for swimming and diving.
Birds are very muscular animals. Most of the muscles are concentrated in the breast and upper leg. The neck contains a mass of thin, stringy, interwoven muscles. The structure of this muscle mass is such that the neck is very flexible in its movements, permitting a bird to rotate its head to a greater extent than most other vertebrates.
The breast of a bird contains one of two types of skeletal muscle: red muscle or white muscle. Red muscles obtain their color from a network of capillaries that provide them with oxygen-rich blood. The muscles are composed of thin fibers that are rich in myoglobin, an oxygen-carrying compound. Oxygen is extracted from the blood and transported by the myoglobin to the muscle fibers where, by the chemical processes called metabolism, energy is released from protein, the main source of energy. Most birds have red breast muscles. Birds that are strong fliers over long distances, such as falcons, gulls, and sparrows, have very well-developed red breast muscles.Birds of Australia and New Zealand vary in size and shape.
The circulatory system of birds is similar to that of mammals. The bird has a four-chambered heart, but it is larger and more muscular than that of a mammal of comparable size. As a result, the circulation is more rapid and the blood pressure is higher than in most vertebrates. Birds are generally more active than mammals and have a higher rate of metabolism; they must eat large quantities of food to provide adequate energy. They also have a higher body temperature than most mammals. The body temperature ranges from 102° to 110° F. (39° to 43° C.).
A bird's lungs are small and rigid. A complex system of air sacs is distributed throughout the body and is connected by tubes to the lungs and to some of the hollow bones. Unlike mammals, birds expel all the air from the lungs with each breath. As a result, the lungs are continuously bathed with fresh air. Birds, unlike mammals, have no sweat glands and thus cannot perspire to cool the body after strenuous exercise. Instead, cool air is distributed from the air sacs to adjacent organs, preventing them from overheating.
Birds have a keen sense of sight. Their eyes are large and produce large, sharp images, especially helpful during flight. Most nocturnal birds have larger eyes than birds active during the day.
Birds that dive underwater, such as ducks, loons, and auks, have a transparent membrane (the nictating membrane) that covers the eye while the bird is submerged, allowing it to see underwater. Birds that inhabit open areas, such as hawks, terns, and swallows, have a horizontal streak across their retina. This area contains millions of densely packed visual cells. It lies parallel to the horizon when the head is held in its normal position. The horizontal streak allows for sharp scanning of the horizon without movement of the eyes or neck.
Most birds have both monocular and binocular vision. In monocular vision, the field of view of the two eyes does not overlap—each eye sees a different area. In binocular vision, the field does overlap, creating three-dimensional vision, which is important in judging distances. Binocular vision is essential for hunting fast-moving prey, and is most developed in birds with eyes located toward the front of the head, such as the owl and hawk. These birds rarely use their monocular vision, which covers only a narrow field. Monocular vision is the main form of vision in birds with eyes located on the side of the head, such as songbirds and water fowl. It allows them to see far to the side to detect predators. Since they eat slow-moving or inert objects, they require only a narrow field of binocular vision.
Birds have an acute sense of hearing. Their ears are small openings in the skull and are protected by feathers. Birds are able to hear sounds of frequencies much higher than those humans can perceive. This ability helps them locate burrowing insects and other prey that emit high-frequency sounds.The African lammergeyer uses its excellent vision to find carrion.