Not much is known of the habits of this cuckoo; but what we do know shows that, sometimes, at any rate, it makes the king-crow act as its nursemaid. Mr. Davison saw two king-crows feeding a young Surniculus. The consequence is that every book on natural history trots out our friend the drongo cuckoo as an example of mimicry. The mimicry is, of course, unconscious: it is said to be the result of the action of natural selection.
King-crows are, as every one knows, exceedingly pugnacious birds; at the nesting season both cock and hen are little furies, who guard the nursery most carefully and will not allow a strange species to so much as perch in the tree in which it is placed.
It is thus obvious that the cuckoo who elects to victimise a king-crow is undertaking a “big thing,” yet this is what Surniculus does. It accomplishes its aim by trickery; it becomes a gay deceiver, disguising itself like its dupe. Now I readily admit that the disguise may be of the utmost use to the Surniculus; I can well understand that natural selection will seize hold of the disguise when once it has been donned and possibly perfect it; but I cannot see how natural selection can have originated the disguise as such.
The drongo cuckoo may be called an ass in a lion’s skin, or a lion in an ass’s skin, whichever way one looks at things. When once the skin has been assumed natural selection may modify it so as better to fit the wearer; but more than this it cannot do.
I do not pretend to know the colour of the last common ancestor of all the cuckoos, but I do not believe that the colour was black. What, then, caused Surniculus lugubris to become black and assume a king-crow-like tail?
A black feather or two, even if coupled with some lengthening of the tail, would in no way assist the cuckoo in placing its egg in the drongo’s nest. Suppose an ass were to borrow the caudal appendage of the king of the forest, pin it on behind him, and then advance among his fellows with loud brays, would any donkey of average intelligence be misled by the feeble attempt at disguise? I think not. Much less would a king-crow be deceived by a few black feathers in the plumage of a cuckoo.
I do not believe that natural selection has any direct connection with the nigritude of the drongo cuckoo. It is my opinion that, so far as the struggle for existence is concerned, it matters little to an animal what its colour be. Every creature has to be some colour: what that actual colour is must depend upon a great many factors; among these we may name the metabolic changes that go on inside the animal, its hereditary tendencies, sexual selection, and natural selection. Is it natural selection that has caused the king-crow to be black? I trow not.
The drongo is black because it is built that way; its tendency is to produce black feathers. Just as some men tend to put on flesh, so also some species of birds tend to grow black plumage. In the case of the king-crow sexual selection has possibly contributed to the bird’s nigritude. It is possible that black is a colour that appeals to king-crow ladies. “So neat, you know; a bird always looks well in black, and a forked tail gives him such an air of distinction.”
As the hen drongo is a bird capable of looking after herself, even when incubating, there is no necessity for her to be protectively coloured. As I have repeatedly declared, one ounce of good solid pugnacity is a better weapon in the struggle for existence than many pounds of protective colouration.
Again, in the case of king-crows nigritude may be an expression of vigour, the outward and visible sign of strength.