Many aspects of the bird skeleton reflect the special demands of flight.  For example, note the extensive fusion of vertebral elements.  The posterior thoracic, all of the lumbar and sacral and the anterior caudal vertebrae have been fused into a single structure called the synsacrum (5) The posterior caudal vertebrate are fused into the pygostyle (6), which supports the tail. Note the small, forward-pointing projections called uncinate processes (4) that connect one rib with the preceding one. These structures serve to provide further support and rigidity for the airframe.

Note that the clavicles are fused together to form the furcula (2)  ("wishbone") and that the large sternum has a well-developed keel (3), which provides a large surface area for the attachment of the major flight muscles. Finally, observe the articulation of the avian jaw (which is identical to that of the reptiles from which they are descended).  Although not visible as a separate element, the articular of the lower jaw articulates with the quadrate bone (1) on the skull.  In mammals, the dentary bone (mandible), the only bone in the lower jaw, articulates with the squamosal bone (a portion of the temporal bone in humans).  No longer needed with this new jaw articulation, the articular and quadrate bones migrate into the middle ear to become the malleus and incus respectively, two of the three mammalian ear ossicles that conduct sound from the tympanum to the inner ear.