How wonderful it is that this parental instinct, so beautiful in its
perfect simulation of the action of the bird that has lost the power of
flight, should be found in so large a number of species! But when we
find that it is not universal; that in two closely-allied species one
will possess it and the other not; and that it is common in such
widely-separated orders as gallinaceous and passerine birds, in pigeons,
ducks, and waders, it becomes plain that it is not assignable to
community of descent, but has originated independently all over the
globe, in a vast number of species. Something of the beginnings and
progressive development of this instinct may be learnt, I think, by
noticing the behaviour of various passerine birds in the presence of
danger, to their nests and young. Their actions and cries show that they
are greatly agitated, and in a majority of species the parent bird flits
and flutters round the intruder, uttering sounds of distress. Frequently
the bird exhibits its agitation, not only by these cries and restless
motions, but by the drooping of the wings and tail--the action observed
in a bird when hurt or sick, or oppressed with heat. These languishing
signs are common to a great many species after the young have been
hatched; the period when the parental solicitude is most intense. In
several species which I have observed in South America, the languishing
is more marked. There are no sorrowful cries and restless movements; the
bird sits with hanging wings and tail, gasping for breath with open bill
--in appearance a greatly suffering bird. In some cases of this
description, the bird, if it moves at all, hops or flutters from a
higher to a lower branch, and, as if sick or wounded, seems about to
sink to the ground. In still others, the bird actually does drop to the
ground, then, feebly flapping its wings, rises again with great effort.
From this last form it is but a step to the more highly developed
complex instinct of the bird that sinks to the earth and flutters
painfully away, gasping, and seemingly incapable of flight.