Because such change increases the warmth of the birds ; the white colour reflecting heat most readily, and suffering it to escape but slowly by radiation.
The completeness of this provision for the changes of the season, is beautifully illustrated by the deep colours of the summer dress of birds, being exchanged for the white colour of winter, with a rapidity and extent proportional to the changes of the seasons. During a mild autumn, the shifting of the dark for the light-coloured dress proceeds at a very slow pace; and when the winter also continues mild, the white dress is never fully assumed. In some species, as the black guillemot, the white winter dress is never acquired in this climate, although its ash-coloured plumage intimates a tendency to the change. In the climate of Greenland, on the other hand, the change is complete, and the plumage is of a snowy whiteness.* - Fleming.
Because, if their white dress concealed them from their enemies, these last, by being deprived of their ordinary food, would be in danger of starvation and death. Neither is this variation of colour confined to weak or defenceless animals. Beasts and birds of prey are likewise subject to the change. Hence, if it yielded protection to some, it would enable others to prey with greater certainty of success on their defenceless neighbours ; and would not thus be consistent with the wisdom and benevolence of the Creator. The popular opinion on the subject must, therefore, be relinquished; especially as the change of colour, from dark to white, does not vary, however different the habits, or even stations, of the animals may be, in which it takes place. - Fleming.
Similar changes occur in Quadrupeds: - see Part II. p. 8, of the present work ; and for the difference of dark and white clothing, generally, Parti, p. 59.
Because, when flying in the air, and meeting with many obstacles, as branches and leaves of trees, birds require to have their eyes sometimes as flat as possible, for protection; but sometimes as round as possible, that they may see the small objects, (flies and other insects) which they are chasing through the air, and which they pursue with the most unerring certainty. This could only be accomplished by giving them a power of suddenly changing the form of their eyes. Accordingly, there is a set of hard scales placed on the outer coat of their eye, round the place where the light enters ; and over these scales are drawn the muscles or fibres by which motion is communicated; so that, by acting with these muscles, the bird can press the scales, and squeeze the natural magnifier of the eye into a round shape, when it wishes to follow an insect through the air; and can relax the scales, in order to flatten the eye again, when it would see a distant object, or move safely through leaves and twigs. This power of altering the shape of the eye is possessed by birds of prey in a very remarkable degree. They can see the smallest objects close to them, and can yet discern larger bodies at vast distances, as a carcass stretched upon the plain, or a dying fish afloat on the water. A singular provision is made for keeping the surface of the bird's eye clean, for wiping the glass of the instrument, as it were, and also for protecting it, while rapidly flying through the air, and through thickets, without hindering the sight. Birds are, for these purposes, furnished with a third eye-lid, a fine membrane or skin, which is constantly moved very rapidly over the eyeball by two muscles placed in the back of the eye. One of the muscles ends in a loop, the other in a string which goes through the loop, and is fixed in the corner of the membrane, to pull it backward and forward.