r calls of birds, and even to the
singing of the males. These may well have originated merely as a means
of recognition between the two sexes of a species, and as an invitation
from the male to the female bird. When the individuals of a species are
widely scattered, such a call must be of great importance in enabling
pairing to take place as early as possible, and thus the clearness,
loudness, and individuality of the song becomes a useful character, and
therefore the subject of natural selection. Such is especially the case
with the cuckoo, and with all solitary birds, and it may have been
equally important at some period of the development of all birds. The
act of singing is evidently a pleasurable one; and it probably serves as
an outlet for superabundant nervous energy and excitement, just as
dancing, singing, and field sports do with us. It is suggestive of this
view that the exercise of the vocal power seems to be complementary to
the development of accessory plumes and ornaments, all our finest
singing birds being plainly coloured, and with no crests, neck or tail
plumes to display; while the gorgeously ornamented birds of the tropics
have no song, and those which expend much energy in display of plumage,
as the turkey, peacocks, birds of paradise, and humming-birds, have
comparatively an insignificant development of voice. Some birds have, in
the wings or tail, peculiarly developed feathers which produce special
sounds. In some of the little manakins of Brazil, two or three of the
wing-feathers are curiously shaped and stiffened in the male, so that
the bird is able to produce with them a peculiar snapping or cracking
sound; and the tail-feathers of several species of snipe are so narrowed
as to produce distinct drumming, whistling, or switching sounds when the
birds descend rapidly from a great height. All these are probably
recognition and call notes, useful to each species in relation to the
most important function of their lives, and thus capable of being
developed by the agency of natural selection.

_Decorative Plumage of Birds and its Display._

Mr. Darwin has devoted four chapters of his _Descent of Man_ to the
colours of birds, their decorative plumage, and its display at the
pairing season; and it is on this latter circumstance that he founds his
theory, that both the plumage and the colours have been developed by the
preference of the females, the more ornamented males becoming the
parents of each