Well now, we come back to the question, why, if all species have a serviceable recognition call, that call should not be sufficient for the purpose, just as, without a doubt, it is adequate for all purposes at other seasons? The answer is, I think, clear. The recognition call is not confined to one sex, nor only to breeding birds; it is the common property of all the individuals of the species, and if the female were to rely upon it as a guide she might at one moment pursue another female, at another a non-breeding male; she might even be guided to a paired female or to a paired male, and time would be wasted and much confusion arise. So that no matter how much a male might advertise himself by cries and calls which were common alike to all the individuals of the species, it would not assist the biological end which we have in view. Something else is therefore required to meet the peculiar circumstances, some special sound bearing a definite meaning by which the female can recognise, amongst the host of individuals of no consequence to her, just those particular males in a position to breed and ready to receive mates. Hence the vocal powers, the power of producing sounds instrumentally, and the power of flight, have been organised to subserve the biological end of "recognition."
And this view is strengthened, it seems to me, by the erratic behaviour of certain species, more particularly by one remarkable case, the case of the Cuckoo. The male, after having established himself, utters his call persistently from the day of arrival until approximately the middle of June; but, in contrast with the large majority of species, the female has a characteristic call which she, too, utters at frequent intervals. The female is polyandrous and has a sphere of influence embracing the territories of a number of males; she wanders from place to place, is often silent, and not unfrequently is engaged in dealing with her egg or in searching for a nest in which to deposit it, and therefore she is not always in touch with a male, still less with any particular one. Now there is much evidence to show that the discharge of the sexual function amongst birds is subject to control, and that this control operates through the female--through her physiological state becoming susceptible to stimulation only at certain periods. So that we have these considerations, that the female is polyandrous, that she has a territory distinct from that of the male, and that her sexual impulse is periodical; and the further consideration that the impulse, since it is periodical, is of limited duration and must receive immediate satisfaction. Such being the circumstances of the case, would the voice of the male serve to insure the union of the sexes at the appropriate moment? Well, the fact that she is polyandrous implies that every male in her sphere of influence is not always capable of satisfying her sexual instinct. Is, then, the male's call an indication of his readiness to yield to stimulation? Without a doubt it is an index of the general physiological state which generates the sexual impulse, without a doubt it denotes a general preparedness to breed, but there is no evidence to show that it denotes the degree of ardour of the male at any particular moment, and much that proves the contrary. So that only by the female producing some special sound which will attract the males that are eager and bring them rapidly to the spot where she happens to be, only thus is it possible to insure the consummation of the sexual act. This, it seems to me, is the purpose of the peculiar call of the female--a call which, so far as biological interpretation is concerned, is just as much a song as the melody of the Marsh-Warbler--and its interest for us just now lies in this, that here we have a special case in which the sexes have separate territories, the female is polyandrous, and the voice of the male is not sufficient by itself to bring to pass the union of the sexes; and in which, consequently, if the purpose of song be that of recognition, we should expect to find, as we do find, that the female had a distinct and penetrating call.
