Ornithological experience led me some time back to the belief that at the nesting season a bird becomes a creature of instinct, an organism whose actions are, for the time being, those of a machine, a mere automaton. This view, which has been set forth in the preceding article, is not held by all naturalists. I therefore determined to undertake a systematic series of experiments with a view to putting it to the test. In other words, I decided to play cuckoo. I selected the Indian crow (_Corvus splendens_) as the subject of my experiments, because it is the most intelligent of the feathered folk. If it can be proved that when on the nest the actions of this bird are mechanical, it will follow that the less intelligent birds are likewise mere automata when incubating. Another reason for selecting the crow as my victim is that I have been investigating the habits of the koel (_Eudynamis honorata_), which is parasitic on the crow, and in so doing have had to visit a large number of crows’ nests.
The crow lays a pale blue egg blotched with brown, while the egg of the koel is a dull olive-green also blotched with brown. It is considerably smaller than the crow’s egg. I have seen dozens of koel’s eggs, but never one that a human being could possibly mistake for that of a crow, yet our friend _Corvus_ is unable to detect the strange egg when deposited in the nest and sits upon it. It is not that birds are colour-blind. The koel is able to distinguish its own egg from that of the crow, for, after it has deposited its egg, it frequently returns to the nest and removes one or more of the crow’s eggs! I am convinced that ordinarily a crow would have no difficulty in distinguishing between the two kinds of egg; but at the nesting time it throws most of its intelligence to the winds and becomes a puppet in the hands of its instincts, which are to sit upon everything in the nest.
I have myself placed koel’s eggs in crows’ nests, and in every case the crow has incubated the eggs. On one occasion I came upon a crow’s nest containing only two koel’s eggs. As the nest was some way from my bungalow and in an exposed situation, I knew that, the moment I left, it would be robbed by some mischievous native boy, so I took the eggs and placed them in a crow’s nest in my compound. This already contained three crow’s eggs, two of which I moved, substituting the koel’s eggs for them. The crow’s eggs had only been laid three or four days, but the koel’s eggs were nearly incubated, since both yielded chicks on the third day after I placed them in the nest. If nesting crows think, that pair must have been somewhat surprised at the speedy appearance of the chicks!