The bird skeleton is highly adapted to the capacity for flight. It is extremely lightweight but strong enough to withstand the stresses that a bird experiences, when taking off, flying or landing. One of the adaptations that make this possible is the fusing of bones that are separate in mammals, into single ossifications, such as the pygostyle. Because of this, birds usually have a smaller number of bones than mammals or reptiles.
Birds have a jaw that has adapted into a beak, on which baby birds have an egg tooth.
Birds have many bones that are hollow, with criss-crossing struts or trusses (cross walls) for structural strength. (Some flightless birds like penguins have only solid bones, however). The number of hollow bones varies from species to species, though large gliding and soaring birds tend to have the most. Most bones contain oxygen which also makes them lighter. Birds also have more cervical (neck) vertebrae than many other animals; most have a highly flexible neck that consists of 13-25 vertebrae. Birds are the only vertebrate animals to have a fused collarbone (the furcula or wishbone) or a keeled breastbone.
The skull consists of five major bones:
- Frontal (top of head).
- Parietal (back of head).
- Premaxillary and Nasal (top beak).
- Mandible (bottom beak).
The skull of a normal bird usually weighs about 1% of the birds total bodyweight.
Neck, back, and tail
The vertebral column consists of vertebrae, and is divided into three sections:
- Cervical (13-16) (neck).
- Synsacrum (fused vertebrae of the back, also fused to the hips (pelvis)).
- Pygostyle (tail).
The chest consists of the furcula (wishbone) and coracoid (collar bone), which two bones, 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 (see The Chest), 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 (fingers) 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:
- Illium (top of the hip).
- Ischium (sides of hip).
- Pubis (front of the hip).
These are fused into one (the innominate bone). They meet at the acetabulum (the 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. This aids in flight.