At the breeding season intelligence, which counsels self-preservation, gives way before the parental instinct, which causes birds to expose themselves to danger, and, in some cases, even to sacrifice their lives for the sake of their offspring.
From the construction of the nest until the time when the young ones are fledged the actions of the parent birds are, at any rate in the neighbourhood of the nest, those of automata, rather than of creatures endowed with intelligence.
On this hypothesis alone are many of the actions of nesting birds comprehensible.
That the construction of the nest is in the main an instinctive habit and not the result of intelligence is proved by the fact that a bird which has been hatched out in an incubator will, at the appointed season, build a nest. If birds were not guided by instinct they would never take the trouble to do such a quixotic thing. What benefit can they derive from laboriously collecting a number of twigs and weaving them into a nest?
It is, of course, natural selection that has originated this instinct; for those species in which the parental instinct is not developed, or in which there is not some substitute for it, must inevitably perish. When once this instinct has taken root natural selection will tend to perpetuate it, since those species which take the best care of their young are those which are likely to survive in the struggle for existence.
Many instances can be adduced to show how automatic are the actions of birds at the nesting season.
It sometimes happens that a bird lays an egg and then proceeds to build a nest on top of it.
Again, some birds do not know their own eggs. A whole clutch of different ones may be substituted for those upon which the bird is sitting and the bird will not discover the change.
The well-known bird-photographer, Mr. R. Kearton, was desirous of obtaining a good photograph of a sitting thrush, and as he was afraid that her eggs would be hatched before a fine, sunny day presented itself, had some wooden dummies made. These he painted and varnished to look like those of the thrush, and put them in the nest, wondering whether the bird would be deceived. He need not have wondered; she would probably have sat upon the shams even had they not been coloured.