So far as appearances go this incorrigible mimic is easily holding his own in "the struggle for existence." If his mimicry causes confusion among any of the species mimicked it apparently does not come back on him or his race. His mimicry, so far as I, myself, have ever observed, or read, or heard, does not cause confusion among his own kind. But I doubt if he gains anything beyond a certain "aesthetic pleasure"—an emotion doubtless shared with his mate, but not otherwise having any "social" significance.
It might be claimed, of course, that the Mockingbird "gets away with" his promiscuous mockery solely because of his pugnacious disposition and ability to take care of himself, whereas a less able bird, say a Goldfinch, might court death if it had the temerity to imitate a Sparrow Hawk or a Shrike. Personally, however, I cannot easily picture a Goldfinch[1] (assuming it to be an imitative bird) meeting its death as the result of mocking a Shrike, The Shrike is not a gregarious bird, and the only call it might conceivably respond to (that is, fly towards) would be the sex-call of its kind. It would then respond in a wooing, not in a killing, mood.
Let me concede the possibility, however, that it might, on discovering its mistake, change its mood, and kill the Goldfinch. Such events would tend to eliminate from the racial song of the Goldfinch any and all notes copied from birds that prey on the Goldfinch, since, in each case, the individual doing the imitating would perish. But even though this may be the case with some birds, it does not seem to be with the Mockingbird, whom we may observe imitating "dangerous" birds with apparent impunity.
It seems to me utterly absurd that a Shrike should "imitate other birds for the purpose of attracting them within range of its attack"—a habit that Yarrell, as quoted by Witchell (pp. 173-174), attributes to the Great Grey Shrike. It very well may be that the Great Grey Shrike has imitative powers; and undoubtedly this bird is capable, in certain ways, of profiting by chance experience. But I cannot picture this as one of the ways: it is endowing the bird with too much intelligence.
I do not believe, then, that practical or economical considerations enter into the choice of sounds to be imitated by the Mockingbird or by any bird. The behavior of imitating birds does not indicate such to be the case. There are two main factors, however, that I believe to be reasonably borne out by observable facts, influencing the choice of sounds to be imitated. In discussing these factors, let us return to the Brown Towhee as being a bird in the simpler and early stages of song evolution and as furnishing therefore a less complex "test case" for our purposes than the Mocker. To make our case definite let us keep in mind as our individual example the Brown Towhee of the Chalk Peak region of the Santa Lucias who imitated the Western House Wren. Let us make a note of the fact that he belongs to a non-migratory race. The association where I found him was arid Transition in the yellow pine belt. It is assumed that a bird is most influenced by the sounds of its enviromnent during its own song period, that is, during the spring and summer months. With these ideas in mind, it can be demonstrated, I believe, that the number of bird-notes available for imitation in the habitat of our Brown Towhee is far more limited
- ↑ Since there are no American birds besides the Mockingbird in the evolved stage of mimicry, I am forced to choose as my example a non-imitative bird. In contrast to the derth of mimics in this country see Witchell's enumeration of imitative British birds (pp. 190-229).
