Many facts have already been given which show that birds do adapt their nests to the situations in which they place them; and the adoption of eaves, chimneys, and boxes by swallows, wrens, and many other birds, shows that they are always ready to take advantage of changed conditions. It is probable, therefore, that a permanent change of climate would cause many birds to modify the form or materials of their abode, so as better to protect their young.[172]
In America the change of habits in this respect undergone by the house-swallow has been accomplished within the last three hundred years.
Closely connected, if not identical, with this fact is another, namely, that in some species which have been watched closely for a sufficient length of time, a steady improvement in the construction of nests has been observed. Thus C. G. Leroy, who filled the post of Ranger of Versailles about a century ago, and therefore had abundant opportunities of studying the habits of animals, wrote an essay on 'The Intelligence and Perfectibility of Animals from a Philosophical Point of View.' In this essay he has anticipated the American observer Wilson in noticing that the nests of young birds are distinctly inferior to those of older ones, both as regards their situation and construction. As we have here independent testimony of two good observers to a fact which in itself is not improbable, I think we may conclude that the nest-making instinct admits of being supplemented, at any rate in some birds, by the experience and intelligence of the individual. M. Pouchet has also recorded that he has found a decided improvement to have taken place in the nests of the swallows at Rouen during his own lifetime; and this accords with the anticipation of Leroy that if our observation extended over a sufficient length of time, and in a manner sufficiently close, we should find that the accumulation of intelligent improvements by individuals of successive generations would begin to tell upon the inherited instinct, so that all the nests in a given locality would attain to a higher grade of excellence.
Leroy also says that when swallows are hatched out too late to migrate with the older birds, the instinct of migration is not sufficiently imperative to induce them to undertake the journey by themselves. 'They perish, the victims of their ignorance, and of the tardy birth which made them unable to follow their parents.'
Cuckoo.
Perhaps the strangest of the special instincts manifested by birds is that of the cuckoo laying its eggs in the nests of other birds. As the subject is an important one from several points of view, I shall consider it at some length.
It must first be observed that the parasitic habit in question is not practised by all species of the genus—the American cuckoo, for instance, being well known to build its nest and rear its young in the ordinary manner. The Australian species, however, manifests the same instinct as the European. The first observer of the habit practised by the European cuckoo was the illustrious Jenner, who published his account in the 'Philosophical Transactions.'[173] From this account the following is an extract:—
The cuckoo makes choice of the nests of a great variety of small birds. I have known its eggs entrusted to the care of the hedge-sparrow, water-wagtail, titlark, yellowhammer, green linnet, and winchat. Among these it generally selects the three former, but shows a much greater partiality to the hedge-sparrow than to any of the rest; therefore, for the purpose of avoiding confusion, this bird only, in the following account, will be considered as the foster-parent of the cuckoo, except in instances which are particularly specified.
When the hedge-sparrow has sat her usual time, and disengaged the young cuckoo and some of her own offspring from the shell,[174] her own young ones, and any of her eggs that remain unhatched, are soon turned out, the young cuckoo remaining possessor of the nest, and sole object of her future care. The young birds are not previously killed, nor are the eggs demolished, but all are left to perish together, either entangled about the bush which contains the nest, or lying on the ground under it.
On June 18, 1787, I examined the nest of a hedge-sparrow, which then contained a cuckoo's and three hedge-sparrow's eggs. On inspecting it the day following, I found the bird had hatched, but that the nest now contained a young cuckoo and only one young hedge-sparrow. The nest was placed so near the extremity of a hedge, that I could distinctly see what was going forward in it; and, to my astonishment, saw the young cuckoo, though so newly hatched, in the act of turning out the young hedge-sparrow.