Structural adaptations for locomotion are admirably illustrated in the bird, which typically has two methods of locomotion—walking and flying.
Adaptations in Connection with Flight
The streamlined shape and lightly built body of the bird are obvious adaptations for flying. The weight of the body is largely decreased owing to the peculiar structure of the bones, which frequently show a spongy internal tissue. The central cavities of the long bones are large and often without marrow, while many contain air sacs which are connected with the lungs. The wing feathers greatly increase the striking surface of the wings, but do not add greatly to their weight. A further adaptive feature is the reduction in the number of digits in the hand, and the fusion of certain bones in the wrist and arm. Since the fore limb has lost its grasping and walking function and has become merely a skeletal axis for the attachment of feathers, the full number of digits are not required and would only add unnecessarily to the weight of the body. It is essential that the wing should be kept stiff during flight, and that the parts corresponding to upper arm, forearm and hand should move as a whole. It is in order to maintain this rigidity that the fusion of certain bones in the wing has taken place. Thus the radius and ulna cannot twist about each other during flight, and the range of possible movement in the hand is much reduced by the fusion of all the carpal bones except two with the metacarpals. There is sufficient movement at the joints, however, to enable the wing to be folded compactly to the sides of the body when not in use.
For efficient flight the wings must be firmly joined to the rest of the skeleton, and accordingly the shoulder girdle of a flying bird is well developed. The scapula is fixed to the ribs and backbone by ligaments, and the coracoid is a strong hone connecting the shoulder girdle with the sternum. The clavicles, which form the wish-bone, are also fixed to the sternum, and if they are broken the bird cannot fly. The sternum is fixed to the lower end of the ribs, which are entirely of bone and are not cartilaginous as is the case in mammals. Each rib has a backward projection which overlaps the rib behind, so that together the ribs form a firm framework. Thus the ribs, sternum and shoulder girdle provide a rigid foundation to which the wings are fixed, and at the same time form an efficient protection for the internal organs.
The Wing Muscles
The wings are moved by powerful muscles which form the breast of the bird. A large surface is required for the attachment of these muscles, and accordingly the sternum of a bird is a broad, shield-shaped bone which is much larger than the narrow rod-like sternum of a running animal such as the rabbit. The sternum of a bird has a keel-like projection in front which further increases the area to which muscles may be attached. The outer part of the breast is the large pectoralis major muscle, which passes up to the shoulder and is attached to the lower surface of the humerus. Contraction of this muscle pulls the wing down. Beneath the pectoralis major lies the pectoralis minor, which lifts the wing and which is a smaller muscle since less power is required for the upstroke than for the downstroke. The tendon of the pectoralis minor threads through a hole, the foramen triosseum, formed by the meeting of the three bones of the shoulder girdle, and is attached to the upper surface of the humerus. Thus, although the muscle is below the wing, the foramen triosseum acts as a pulley over which the tendon passes, and consequently contraction of the muscle lifts the wing.
Adaptations in Connection with Walking
The habit of walking on two legs is the necessary consequence of the adaptation of the fore legs of the bird for flight, and the bird ’s skeleton shows many features which are related to this method of walking. The skeleton is remarkable for the extent to which fusion has taken place among the vertebras, and for the great length of the part of the pelvic girdle which is attached to the backbone. In reptiles which walk on four legs the weight of the body is carried by all the legs, and the attachment of the pelvic girdle to two vertebras only provides sufficient support for the hind part of the body. In birds and in man the weight of the body is balanced on two legs, but owing to the upright posture of man the weight of the body is above the pelvic girdle, whereas in a bird the body leans forward from the legs and the centre of gravity lies in front of the acetabulum. In order to hold the body in thisthe pelvic girdle is firmly fixed to eighteen or nineteen vertebra, which are themselves fused together to form the structure known as the synsacrum.
A bird ’s leg is not straight but bent so that the thigh bone runs forward from the hip and the tibio-fibula slopes backward from its junction with the thigh bone. This arrangement of the bones helps the bird to maintain its balance while walking, and enables it to spring easily from the ground when taking to flight. It also lessens the shock that the bird receives on landing. Strength and firmness are given to the bird ’s leg by the absence of all free tarsals, of which the proximal row have fused with the tibio-fibula, and the distal row with the metatarsals to form the tarso-metatarsal bone. This bone represents the three fused metatarsals, which have become raised from the ground so that the bird walks on the end joints of its toes.
The Perching Mechanism.
In the ostrich, cassowary and emu the power of flight has been lost, and in these birds many of the adaptive features of flying birds are not present. The feathers are no longer required to form a firm surface to beat the air, and have become soft and downy owing to the loss of barbules. Since the pectoral muscles are not developed for flight, the keel of sternum of flightless birds has become very small or has degenerated and the bones are small, while the wing itself is much reduced.
In most swimming birds the motive power is supplied by the feet, which act as paddles and are placed nearer the hinder end of the body than in birds which do not swim. The legs are widely separated from each other and give the bird an ungainly gait when walking. The toes are united by webs of skin, and are stretched apart during the backward stroke when the bird is swimming, but during the forward stroke are closed together so as to offer less resistance to the water. Resistance to the water is also lessened during the forward stroke owing to the fact that the tarso-mctatarsus is flattened, but during the backward stroke this bone is partly rotated so that the flattened side adds power to the stroke.
Penguins are flightless birds which, however, do not show the skeletal features of the ostrich and the emu, for their flipper-like wings are used in swimming and diving. Consequently penguins resemble flying birds in possessing a sternum with a strong keel and a well-developed shoulder girdle. 14 A