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.

Why do birds sing?

Because of the receptacles of air already mentioned but particularly by the disposition of the larynx, which in birds is not, as in mammifera and amphibia, placed wholly at the upper end of the windpipe; but, as it were, separated into two parts, one placed at each extremity.  Parrots, ravens, starlings, bullfinches, &c., have been taught to imitate the human voice, and to speak some words:  singing birds also, in captivity, readily adopt the song of others, learn tunes, and can even be made to sing in company, so that it has been possible actually to give a little concert by several bullfinches.  In general, however, the song of birds in the wild state appears to be formed by practice and imitation.—­Blumenbach.

Why do the notes of different species of birds vary?

Because, probably, of the structure of the organs of each species enabling them more easily to produce the notes of their own species, than those of any other, and from the notes of their own species being more agreeable to their ears.  These conditions, joined to the facility of hearing the song of their own species, in consequence of frequenting the same places, determine the character of the acquired language of the feathered tribes.—­Fleming.

Why are birds equally dispersed in spring over the face of the country?

Because, during that amorous season, such a jealousy prevails between the male birds, that they can hardly bear to be seen together in the same hedge or field.  Most of the singing and elation of spirits, of that time, seem to be the effect of rivalry and emulation.—­G.  White.

Why is August the most mute month, the Spring, Summer, and Autumn through?

Because many birds which become silent about Midsummer, reassume their notes in September; as the thrush, blackbird, woodlark, willow-wren, &c.—­G.  White.

Why do birds congregate in hard weather?

Because, as some kind of self-interest and self-defence is, no doubt, their motive, may it not arise from the helplessness of their state in such rigorous seasons; as men crowd together, when under great calamities, they know not why?  Perhaps approximation may dispel some degree of cold; and a crowd may make each individual appear safer from the ravages of birds of prey and other damages.—­G.  White.