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 !
In all, I have placed six koel's eggs in four different crow's nests, and as I have already said, in no single instance did the trick appear to be detected. In the majority of cases, I did not trouble to keep the number of eggs in the nest constant. I merely added the koel's egg to those already in the nest.
But I have put my theory to a much more severe test. In a certain crow's nest containing two eggs I put a large fowl's egg. This was cream-coloured and fully three times the size of the crow's egg, yet within ten minutes the crow was sitting comfortably on the strange egg. She did not appear to notice the considerable addition to her clutch. She subsequently laid three more eggs, so that she had six eggs to sit upon, five of her own and the large fowl's egg! Day after day I visited the nest and watched the progress of the strange egg. On the twentieth day the chick inside was moving, but when I went to the nest on the twenty-first day I discovered that some one had climbed the tree, for several branches were broken. Two young crows had been taken away and the fowl's egg thrown upon the ground. There it lay with a fully formed black chicken inside! I have that chicken in a bottle of spirit. Subsequent inquiry showed that the dhobi's son had taken it upon himself to spoil my experiment. However, it went sufficiently far to prove that crows may one day become birds of economic value; why not employ them as incubators ? Had the crow come across that chick's egg anywhere but in its nest, it would undoubtedly have made its breakfast off it.
I repeated the experiment in another nest. This time the chick hatched out. When it appeared the rage of the crows knew no bounds. With angry squawks the scandalised birds attacked the unfortunate chick, and so viciously did they peck at it that it was in a dying state by the time my climber reached the nest.
With a view to determining at what stage the incubating instinct secures its dominance, I placed another fowl's egg in a crow's nest that was almost ready to receive eggs, wondering whether the presence of this egg would stimulate the crow to lay, without troubling to give the final touches to the nest. The bird devoured the egg. It is my belief that the acts of a nesting bird do not become completely automatic until it has laid an egg in the nest. If one visits a crow's nest which is in course of construction, the owners will as likely as not desert it; but I have never known a crow desert its nest when once it has laid an egg -: provided, of course, he who visits the nest leaves any eggs in it.
In another nest containing two crow's eggs I placed a golf ball; on returning next day I found the crow sitting tight upon her own two eggs and the golf ball!
But in another case, where I had found two eggs and substituted for them a couple of golf balls, the crow refused to sit. I suppose the idea was, " I may be a bit of a fool when I am nesting, but I am not such a fool as all that!" I once came across a young koel and a crow's egg in a nest. I removed the former and placed it in a crow's nest containing four crow's eggs. The owner of the nest showed no surprise at the sudden appearance of the koel, but set about feeding it in the most matter-of-fact way. The young koel was successfully reared ; it is now at large and will next year victimise some crow. I may say that no human being could possibly fail to distinguish between a young koel and a young crow. When first hatched the koel has a black skin, the crow a pink one. The mouth of the crow nestling is an enormous triangle with great fleshy flaps at the side; the mouth of the koel is much smaller and lacks the flaps. The feathers arise very differently in each species, and whereas those of the crow are black, those of the koel are tipped with russet in the cock and white in the hen.
In another nest containing a young koel (put there by me) and two crow's eggs, I placed a paddy bird's (Ardeola grayii) egg, hoping that the gallant crow would hatch it out and appreciate the many-sidedness of her family. She hatched out the egg all right, at least I believe she did. I saw it in the nest the day before the young paddy bird was due; but when I visited the nest the following morning neither egg nor young bird was there. It would seem that the crow did not appreciate the appearance of the latest addition to the family and destroyed it. It is, of course, possible that the young koel declined to associate with such a neighbour and killed it; but I think that the crow was the culprit, for I had previously placed a paddy bird nestling, four days old, in a crow's nest containing only young crows, and the paddy bird had similarly disappeared.
These, then, are the main facts which my game of cuckoo has brought to light. They are not so decisive as I had expected. They seem to indicate that the actions of birds with eggs or young are not quite so mechanical as I had supposed. Were they not largely mechanical a crow would never hatch out a koel's egg, nor would it feed the young koel when hatched out; it would not incubate a fowl's or a paddy bird's egg, and it would assuredly decline to sit upon a golf ball. On the other hand, were the acts of nesting birds altogether mechanical, the young paddy birds would have been reared up, and the substitution of two golf balls for two eggs would not have been detected. There is apparently a limit to the extent to which intelligence is subservient to blind instinct.