Why Do Birds Have Hollow Bones?
Birds fly so well because they have developed skeletons which are especially light and strong. Most of their bones are hollow, with the interior webbed or honeycombed across by fine girders of bone to give added strength. They are sometimes called “pneumatic” or air-filled bones.
The bird skeleton is highly adapted for flight. It is extremely lightweight but strong enough to withstand the stresses of taking off, flying, and landing. One key adaptation is the fusing of bones into single ossifications, such as the pygostyle. Because of this, birds usually have a smaller number of bones than other terrestrial vertebrates.
A bird’s skull is made of thin bone in remarkable contrast to the solid, heavy skull of the mammal. The bones of its spine are flexibly connected in the neck, strongly bound together in the front part of the body and united at the rear into a solid, rigid mass. Powerful muscles attached to the breast bone move the wings.
The bones in the wings have been reduced in number to provide greater strength. Wings can be used also as propellers. They can be shortened or lengthened by flexing, the feathers at the tips can be spread or closed, and the angle of the wings or their parts can be altered.
All these adjustments make the aerodynamics of a bird’s wings much more complicated than those of an aircraft. Consequently, the flight of a bird is more varied and adaptable.
Birds have many bones that are hollow (pneumatized) with criss-crossing struts or trusses for structural strength. The number of hollow bones varies among species, though large gliding and soaring birds tend to have the most. Respiratory air sacs often form air pockets within the semi-hollow bones of the bird’s skeleton.
The bones of diving birds are often less hollow than those of non-diving species. Loons and puffins are without pneumatized bones entirely. Flightless birds, such as ostriches and emus, demonstrate osseous pneumaticity, possessing pneumatized femurs and, in the case of the emu, pneumatized cervical vertebrae.
Birds also have more cervical (neck) vertebrae than many other animals; most have a highly flexible neck consisting of 13-25 vertebrae. Birds are the only vertebrates to have a fused collarbone (the furcula or wishbone) or a keeled sternum or breastbone. The keel of the sternum serves as an attachment site for the muscles used for flight or, similarly, for swimming, in penguins.
Again, flightless birds, such as ostriches, which do not have highly developed pectoral muscles, lack a pronounced keel on the sternum. Swimming birds have a wide sternum, while walking birds have a long or high sternum and flying birds have a sternum width and height that are nearly equal.
Birds have uncinate processes on the ribs. These are hooked extensions of bone which help to strengthen the rib cage by overlapping with the rib behind them. This feature is also found in the tuatara Sphenodon. They also have a greatly elongate tetradiate pelvis, similar to some reptiles.
The hind limb has an intra-tarsal joint found also in some reptiles. There is extensive fusion of the trunk vertebrae as well as fusion with the pectoral girdle. They have a diapsid skull, as in reptiles, with a pre-lachrymal fossa (present in some reptiles). The skull has a single occipital condyle.
The skull consists of five major bones: the frontal (top of head), parietal (back of head), premaxillary and nasal (top beak), and the mandible (bottom beak). The skull of a normal bird usually weighs about 1% of the bird’s total bodyweight. The eye occupies a considerable amount of the skull and is surrounded by a sclerotic eye-ring, a ring of tiny bones. This characteristic is also seen in reptiles.
The vertebral column consists of vertebrae, and is divided into three sections: cervical (11-25) (neck), synsacrum (fused vertebrae of the back, also fused to the hips (pelvis)), and pygostyle (tail).
The chest consists of the furcula (wishbone) and coracoid (collar bone), which, together with the scapula (see below), form the pectoral girdle. The side of the chest is formed by the ribs, which meet at the sternum (mid-line of the chest).
The shoulder consists of the scapula (shoulder blade), coracoid, and humerus (upper arm). The humerus joins the radius and ulna (forearm) to form the elbow. The carpus and metacarpus form the “wrist” and “hand” of the bird, and the digits are fused together. The bones in the wing are extremely light so that the bird can fly more easily.
The hips consist of the pelvis, which includes three major bones: the illium (top of the hip), ischium (sides of hip), and pubis (front of the hip). These are fused into one (the innominate bone). Innominate bones are evolutionary significant in that they allow birds to lay eggs. They meet at the acetabulum (hip socket) and articulate with the femur, which is the first bone of the hind limb.
The upper leg consists of the femur. At the knee joint, the femur connects to the tibiotarsus (shin) and fibula (side of lower leg). The tarsometatarsus forms the upper part of the foot, digits make up the toes. The leg bones of birds are the heaviest, contributing to a low center of gravity, which aids in flight. A bird’s skeleton comprises only about 5% of its total body weight.