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The avian skeleton is notable for its strength and lightness, achieved by fusion of elements and by pneumatization (i.e., presence of air cavities).
The skull represents an advance over that of reptiles in the relatively larger cranium with fusion of elements, made possible by the fact that birds have a fixed adult size.
Birds differ from mammals in being able to move the upper mandible rather than the lower, relative to the cranium. When the mouth is opened, both the lower and upper jaws move: the former by a simple, hingelike articulation with the quadrate bone at the base of the jaw, the latter through flexibility provided by a hinge between the frontal and nasal bones.
As the lower jaw moves downward, the quadrate rocks forward on its articulation with the cranium, transferring this motion through the bones of the palate and the bony bar below the eye to the maxilla, the main bone of the upper jaw. The number of vertebrae varies from 39 to 63, with remarkable variation (11 to 25) within the neck (cervical) series. The principal type of vertebral articulation is heterocoelous (saddle shaped).
