until the parent, by a changed note, conveys to it an intimation that
the danger is over. Another proof that the nestling has absolutely no
instinctive knowledge of particular enemies, but is taught to fear them
by the parents, is to be found in the striking contrast between the
habits of parasitical and genuine young in the nest, and after they have
left it, while still unable to find their own food. I have had no
opportunities of observing the habits of the young cuckoo in England
with regard to this point, and do not know whether other observers have
paid any attention to the matter or not, but I am very familiar with the
manners of the parasitical starling or cow-bird of South America. The
warning cries of the foster parent have no effect on the young cow-bird
at any time. Until they are able to fly they will readily devour worms
from the hand of a man, even when the old birds are hovering close by
and screaming their danger notes, and while their own young, if the
parasite has allowed any to survive in the nest, are crouching down in
the greatest fear. After the cow-bird has left the nest it is still
stupidly tame, and more than once I have seen one carried off from its
elevated perch by a milvago hawk, when, if it had understood the warning
cry of the foster parent, it would have dropped down into the bush or
grass and escaped. But as soon as the young cow-birds are able to shift
for themselves, and begin to associate with their own kind, their habits
change, and they become suspicious and wild like other birds.

On this point--the later period at which the parasitical young bird
acquires fear of man--and also bearing on the whole subject under
discussion, I shall add here some observations I once made on a dove
hatched and reared by a pigeon at my home on the pampas. A very large
ombu tree grew not far from the dove-cote, and some of the pigeons used
to make their nests on the lower horizontal branches. One summer a dove
of the most common species, Zenaida maculata, in size a third less than