Adaptations to Flight

The birds are so well adapted and specialized for aerial mode of life that their whole organization and body structure is based on this mechanism. Since flight necessitates a large surface weight ratio, birds do not become so large as some mammals. Their body is short, fusiform or boat-shaped to offer little resistance to the air. The body is covered with feathers, which are means of heat insulation and flight. There are different kinds of feathers on different parts of the body, meant for different functions. The important of these are the wing-feathers and tail feathers. The wing feathers are carried along the post-axial border of the humerus, ulna and hand. The shape of the wing depends on the position in which the feathers are held by their muscles, as well as on membranes, the pre-and post patagia, developed where the limb joins the body. The tail feathers, like the wing feathers are very large but in birds living near to the ground they are reduced or absent. They are very large in fast moving birds that change direction quickly and are often used for steering purpose. The modification of forelimb into wing is one of the most important feathers for adaptation to flight. The humerus is short and broad, radius and ulna are long, especially the latter. There are only two free proximal carpals; and the remainder of the wrist is formed of three metacarpals, one is short two longs and fused. Only one digit, probably representing the second is well developed, the third and the first are reduced. All these modifications are brought about for bearing the large wing feathers or remiges and large pectoral muscles. The wing is designed to have a minimum moment of inertia about an axis parallel to the sagital plane and passing through the shoulder muscles. Muscles lying on either outside of the arm or in its proximal part, with long tendons produce movements. The huge pectoral muscles, making a greater part of the weight of the body produce the active movements of flight. Depression of the wing is produced mainly by pectoralis major, and elevation of wing is produced by the pectoralis minor. 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.

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