SKELETON OF BIRD

All birds have the same basic plan, though different life styles have meant that they have evolved different variations on the central theme. Birds such as swans have more vertebrae in their necks than those like parrots who have very short necks. Swans have 25 vertebrae, parrots have 9. Most flightless birds, having no need of the large flight muscles, have therefore, no need of the large keel or sternum that flying birds have. The exception to this rule are penguins who effectively fly through the water with their modified wings and still need large muscles to power them.

Bird's inherited their basic skeleton from their reptilian ancestors. The constraints of flight however have meant that birds have had to modify it in several major ways. Flight means lifting the birds weight, so the first major consideration is reduction in weight. The lighter the bird, the easier it is to fly. The main ways birds have lost weight is through the loss of teeth and the large jaw bones needed to support teeth, the loss of nearly all the tail and reduction of the skull. Though a bird's major limb bones are hollow with internal struts for support, this makes them stronger not lighter; a bird's leg bones for example are often heavier than those of similar sized mammal or reptile.

Function part of skeleton :

The bones

While maintaining strength, most of the bones are pneumatic, meaning they are hollow and filled with air spaces connected to the respiratory system.

Skull

The bones of the skull are generally fused providing protection to the brain while being of light weight. A light, toothless beak replaces the bony, heavy toothed jaw of reptiles. Beaks, of course, can be highly modified for different types of food and feeding behavior. Note the large orbits, as sight is an important sensory mechanism for birds.

Neck

The necks of birds are very important for body maintenance and eyesight. Modification for flight has rendered avian forelimbs almost useless for any task other than flight. To make up for this lack of forelimb dexterity, the beak is used for many tasks such as preening feathers. To access hard-to-reach feathers on the back and tail birds require a flexible neck. Furthermore, as birds have immobile eyes, head movement and flexibility is required to focus on objects at various distances.

Thorax and Sternum

Overlying flaps projecting off the ribs called uncinate processes help to stiffen the rib cage so it will not collapse during the powerful strokes required for flight. The sternum is the highly modified breastbone. In flying and swimming birds the keel is enlarged for flight muscle attachment. Flightless birds such as Ostriches have a sternum without a keel.

Pectoral Girdle

The pectoral girdle is made up of the sternum, clavicle, coracoid and scapula. The clavicles come together to form the furcula, or "wishbone". The furcula provides a flexible attachment site for the breast muscles and along with the coracoids act as struts that resist pressure created by the wing stroke during flight. Flight muscles running from the sternum to the relatively short and stiff humerus elevate and depress the wing.

Pelvic Girdle

There is an extensive fusion of bones of the pelvic region to provide stiff support for the legs in order to deal with the stress of take-off and landing. The synsacrum is a fusion of the pelvic and 6 caudal (tail) vertebrae. At the end of the spinal column is the pygostyle, a fusion of the final few caudal vertebrae. The pygostyle supports the tail feathers and musculature.

Wing

The avian wing contains the usual arm bones of reptiles and mammals, but in a highly modified form. The humerus is rather short compared to the total length of the wing, as it must withstand the pulling of the flight muscles. The radius and ulna form the support for the mid-wing. The outer wing or "hand" bones are highly fused for strength and feather support. The first digit or pollex supports the alula, a small feather used to control air flow around the wing.

Leg and Foot

The upper leg is composed of a fairly standard femur, but the lower leg and foot are highly modified by fusion of bones. Of course, between the femur and the fibula and tibiotarsus is the knee, whose location in birds is often confused. The tarsometatarsus is an extended fusion of the foot bones. This lengthening adds extra leverage for running, landing and take-off.