eggs in other birds nests; the slave-making instinct of certain ants;
and the comb-making power of the hive-bee: these two latter instincts
have generally, and most justly, been ranked by naturalists as the most
wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of
the cuckoo 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,
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