Next, the very important structural change was the reduction of specific gravity of the bones and body, and the redistribution of the weight. Most of the weight is placed in lower area of the torso. In order for the bones to lose weight, but not lose structural stability, air-filled cavities developed within many of the skeletal elements, including portions of the skull, neck vertebrae, humerus, and femur. This also bings fresh air directly from the lungs to the cells tissue within the bone.
The adaptation for larger muscle of the keel, breast bone.
The wish bone. This is due to the development of a large breast muscle, used to flap the wings. The wishbone gives strength to the large muscles. If there was no wish bone, the well-worked breast muscles would tear.
and sensory organs are more modified in relation to flight. The birds main
sensory contact is sight, rather than smell like in reptiles and most mammals.
The eyes are large, and sclerotic plates are commonly developed in the
eyeball. The brain and case are vastly enlarged compared to the reptiles,
but this brain expansion is of centres related to sight, rather than in
the cerebral cortex, which is the seat of "intelligence" and
is more highly developed in mammals.