Other muscles running from the body to the humerous produce rotation of the humerus at the glenoid, and adjustment of the patagia. The muscles within the arm itself serve to extend or fold the whole wing and to alter the positions of the parts. The shape of the wing and development of the muscles depend on the nature of the flight. The hind limbs are placed in such a position that they can carry the whole weight of the body when the birds are on the ground. The feet are correspondingly modified for perching, grasping so that birds may readily take off or land whenever there is need for it.
Most characteristic adaptations occur in the skeleton.
Bones are light, and many of them contain extension of the air sacs and are known as pneumatic. Fusion of bones has reached its extreme. Sternum bears a large keel for the attachment of muscles, and the last tail vertebrae are fused to form the pygostyle, supporting the tail feathers. The joints of the vertebral column are reduced so that there can be movement only in the cervical region, between the thoracic and synsacrum and in the tail. By the presence of the keel, with its muscles, the center of gravity is kept below the center of pressure, giving great stability.