If this were the case the process of laying and hatching
might be inconveniently long, more especially as she migrates 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 both asserted and denied that the American cuckoo
occasionally lays her eggs in other birds' nests; but I have lately heard
from Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo,
together with a young jay in the nest of a blue jay (Garrulus cristatus);
and as both were nearly full feathered, there could be no mistake in their
identification. I could also 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, and that she occasionally laid an egg in
another bird's nest. If the old bird profited by this occasional habit
through being enabled to emigrate earlier or through any other cause; or if
the young were made more vigorous by advantage being taken of the mistaken
instinct of another species than when reared by their own mother,
encumbered as she could 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.
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