The axial skeleton, much like the rest of the skeleton, shows close match between form and function.
As with other vertebrates, the ribs of the bird serve as sites of attachment for muscles and function as a protective case. In birds, however, cervical ribs fuse with the vertebrae and are reduced. Many of the ribs of birds bear uncinate processes, which attaches muscles that support the scapula. These processes were probably inherited from their reptile ancestors; fossil as well as some living reptiles also have uncinate processes. These processes overlap ribs to increase rigidity of the thoracic ribs.
The innominate bone and synsacrum are constructed in such a manner as to increase strength and reduce weight. Bones of the pelvic girdle fuse to form the innominate bone. The innominate and synsacrum also fuse together. The synsacrum is a unit formed by the fusion of the posterior thoracic, lumbar, and sacral vertebrae. Fusions of these bones make birds sturdy and indirectly lighter by decreasing the number of muscles necessary to control individual vertebrae.
Birds also have a pygostyle, which is a flat bone created by the fusion of several vertebrae at the end of the column. It serves as an attachment site for tail feathers, which contribute to stabilization during flight.
The sternum of the bird is unique and further demonstrates how the skeletal system of the bird is well-suited. The sternum is a prominent bone which juts out and provides increased surface area for muscle attachment. Because more muscles can attach to the sternum, birds have more strength and control for flight. Most flightless birds have a reduced sternum in comparison to flying birds; these differences also show a close match of form and function.