One key adaptation is the fusing
of bones into
single ossifications, such as
the pygostyle. Because of this, birds usually
have a smaller number of bones than other terrestrial vertebrates. Birds also lack teeth or even a
true jaw, instead having evolved
a beak, which is far more lightweight. The
beaks of many baby birds have a projection called an egg
tooth, which facilitates their exit from the amniotic egg.
Birds have many bones that are hollow (pneumatized) with criss-crossing struts or trusses for structural strength. The number of hollow bones varies among species, though large
gliding and soaring birds tend to have the most. Respiratory air sacs often form air pockets within the semi-hollow
bones of the bird's skeleton. Some flightless birds like penguins and ostriches have only solid bones, further evidencing the link between
flight and the adaptation of hollow bones.
Birds also have more cervical
(neck) vertebrae than many other animals; most
have a highly flexible neck consisting of 13-25 vertebrae. Birds are the only vertebrate animals to have a fused collarbone (the furcula or wishbone) or a keeledsternum or breastbone. The keel of the
sternum serves as an attachment site for the muscles used for flight, or similarly for swimming in penguins.
Again, flightless birds, such as ostriches, which do not have highly developed pectoral muscles, lack a pronounced keel on the sternum. It is noted
that swimming birds have a wide sternum, while walking birds had a long or high sternum while flying birds
have the width and height nearly equal.
Birds have uncinate
processes on the ribs. These are hooked extensions of bone which help to
strengthen the rib cage by overlapping with the rib behind them. This feature is also found in the tuatara
Sphenodon. They also have a greatly elongate
tetradiate pelvis as in some reptiles.
The hindlimb has an intra-tarsal joint found also in some reptiles. There is extensive fusion of the trunk
vertebrae as well as fusion with the pectoral girdle. They have a diapsid skull as in reptiles with a
pre-lachrymal fossa (present in some reptiles). The skull has a single occipital condyle.