uckoo's instinct is, that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own
nest and sit on her own eggs, those first laid would have to be left
for some time unincubated, or there would be eggs and young birds of
different ages in the same nest. If this were the case, the process of
laying and hatching might be inconveniently long, more especially as she
has to migrate at a very early period; and the first hatched young would
probably have to be fed by the male alone. But the American cuckoo is
in this predicament; for she makes her own nest and has eggs and young
successively hatched, all at the same time. It has been asserted that
the American cuckoo occasionally lays her eggs in other birds' nests;
but I hear on the high authority of Dr. Brewer, that this is a mistake.
Nevertheless, I could give several instances of various birds which have
been known occasionally to lay their eggs in other birds' nests. Now let
us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo; but that occasionally she laid an egg in
another bird's nest. If the old bird profited by this occasional habit,
or if the young were made more vigorous by advantage having been taken
of the mistaken maternal instinct of another bird, than by their own
mother's care, encumbered as she can hardly fail to be by having eggs
and young of different ages at the same time; then the old birds or the
fostered young would gain an advantage. And analogy would lead me
to believe, that the young thus reared would be apt to follow by
inheritance the occasional and aberrant habit of their mother, and in
their turn would be apt to lay their eggs in other birds' nests, and
thus be successful in rearing their young. By a continued process of
this nature, I believe that the strange instinct of our cuckoo could be,
and has been, generated. I may add that, according to Dr. Gray and
to some other observers, the European cuckoo has not utterly lost all
maternal love and care for her own offspring.

The occasional habit of birds laying their eggs in other birds' nests,
either of the same or of a distinct species, is not very uncommon with
the Gallinaceae; and this perhaps explains the origin of a singular
instinct in the allied group of ostriches. For several hen ostriches,
at least in the case of the American species, unite and lay first a
few eggs in one nest and then in anoth