Because they have the largest bones of all animals, in proportion to their weight; and their bones are more hollow than those of animals that do not fly. Air-vessels also enable them to blow out the hollow parts of their bodies, when they wish to make their descent slower, rise more swiftly, or float in the air. The muscles that move the wings of birds downwards, in many instances, are a sixth part of the weight of the whole body ; whereas, those of a man are not in proportion one-hundredth part so large.
Because flying is the continued suspension, and progress of the whole body, in the air, by the action of the wings. In leaping, the body is equally suspended in the air, but the suspension is only momentary. In flying, on the contrary, the body remains in the air, and acquires a progressive motion by repeated strokes of the wings on the surrounding fluid. - Fleming.
In swimming on the surface of the water, the legs of birds are exclusively employed ; but when motion is accomplished beneath the surface, the wings are then chiefly in exercise.
Because, by this addition to the non-conducting appendices of the skin, birds are enabled to preserve the heat, generated in their bodies, from being readily transmitted to the surrounding air, and carried off by its motions and diminished temperature. - Fleming.
Because the pinion-feathers may form, when the wing is expanded, as it were, broad fans, by which the bird is enabled to raise itself in the air and fly ; whilst its tail feathers direct its course. - Blumenbach.
Because they have scarcely any pinion-feathers. - Blumenbach.
Because they may be prepared for winter; this change being analogous to the casting of hair in quadrupeds. During summer, the feathers of birds are exposed to many accidents. Not a few spontaneously fall; some of them are torn off during their amorous quarrels ; others are broken or damaged; whilst, in many species, they are pulled from their bodies to line their nests. Hence, their summer dress becomes thin and suitable. Previous to winter, however, and immediately after incubation and rearing of the young is finished, the old feathers are pushed off in succession by the new ones, and thus the greater part of the plumage of the bird is renewed. - Fleming.
Because colour is the gift of light. Thus, the indigo-bird appears at one time of a rich sky-blue, at another of a vivid verdigris green. When the angle of incidence in the rays of light reflected from the plumage of the bird is acute, the colour is green, when obtuse, blue. The colour of the head being of a very deep blue, is not affected by any change of position.