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.

Why are birds covered with feathers?

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._

Why are the strongest feathers of birds in the pinions and tail?

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._

Why do birds moult?

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 stru