The Anatomy of Birds
The structure of most birds is related to their ability to fly and this imposes a number of restrictions on their bone structure and body shape. Because a bird can fly it is desirable that its bones should not only be strong but also light. To achieve this some bones have air spaces or cavities which are crossed by bony struts to strengthen them. Pneumatization, as this is called, occurs chiefly in the wing and other large bones, and even the skull has some of these small air recesses. These cavities are particularly well developed in large gliding and soaring birds such as vultures, albatrosses and eagles but are less so in diving birds such as ducks and cormorants, presumably because any additional bouyancy would require extra energy on the part of the bird when it wanted to dive. Some of the bones have also been strengthened during the course of evolution by the fusion of many of the bones of their reptilian ancestors. On the other hand, the flexibility of the neck has been increased by the large number of vertebrae, ranging from thirteen in the Cuckoo and other song birds to as many as twenty-eight in the swan. This gives the bird freedom to move its head while searching for food, watching for enemies, preening and so on.
The most obvious features of a bird’s skeleton are the skull, the backbone or vertebral column, the pectoral girdle which houses the wing bones, the bones of the pelvic girdle to which are attached the legs, and the ribs which protect the body cavity. The other prominent feature is the breast bone or sternum which has a noticeable keel to which the wing muscles are attached. Generally speaking, the stronger the flyer the deeper the keel; in some flightless birds the keel has entirely disappeared.
I will not describe the entire structure of the bird, but it is worth mentioning some external features. The upper and lower mandibles (the bill) are a bony adaptation of the skull which are covered with a horny sheath. The shape of the bill is usually adapted to a bird’s feeding behaviour, but occasionally as with Puffins and herons the formation of the bill plays an important part in the bird’s displays. Flesh-eating species such as hawks, owls, shrikes and even the fish-eating shearwaters have hooked bills for tearing their prey. Some waders such as Snipe have long thin bills which they use for probing deeply into the mud; the tip is sensitive so that they can detect their prey. Nightjars on the other hand have short, but very wide bills, which enable them to catch flying insects more easily. Even amongst the finches there is a variety of shapes and sizes, from the Goldfinch’s long and thin bill, which is used for poking into Teazel florets, to the twisted bill of the Crossbill, which is used for extracting seed from conifer cones. Birds do not have real teeth although several species have appendages which have the appearance of teeth.
The nostrils are situated in the upper mandible and open into a nasal cavity. In the petrels and a number of related seabirds the nostrils are modified into a horny tube. In Gannets and Cormorants, the nostrils are completely closed and it is assumed that they breathe through their mouth. Legs and toes vary enormously. The four toes are attached to the tarsus and they can be arranged in a variety of ways depending on the methods the bird uses to hunt its food. In most perching birds three toes point forward and one points to the rear. This arrangement suits best the way of life which involves perching on branches and twigs. There is enormous variation in the actual shape of the toes and I feel that the relationship of the feet to the habitat is still a fairly open field for discovery. Woodpeckers which cling to the sides of trees have two claws pointing forwards and two backwards, one of which they can move forward to make three. Walking and running birds like the Lapwing have short toes with longer legs, while other walking birds like the Meadow Pipit have long hind claws, presumably to allow them to walk over the longer grasses, without entangling their feet. Swimming birds need their legs to be located near the hind part of the body where they can best control the steering. The legs of some species are flattened laterally and are articulated so that they can be slipped forward through the water with as little resistance as possible.
The body of a bird has about 175 muscles most of which are paired. The largest and most powerful are the pectoral muscles which in a strong flyer can weigh as much as 30% of the bird’s total weight. These are the muscles which pull the wings downwards and forwards. Muscles cannot push so another muscle which rests below the wing and which is connected to the numerous by a tendon comes into play, acting as a pulley to lift the wing. The legs also have powerful muscles which are used both for locomotion and maintaining the bird’s position when perched. These large muscles in the legs and wings are near the body’s centre of gravity and the main mass of muscle is located, therefore, high over the femur.