Page:Territory in Bird Life by Henry Eliot Howard (London, John Murray edition).djvu/207

be left without their complement of pairs just because this male mistook the voice of that, and avoided it when there was no necessity for doing so. So that just as from the point of view of "recognition" each female must be able to distinguish the voice of its own kind, so likewise the warning can only be adequate providing that the sounds are specifically distinct. A point, however, arises here in regard to closely related forms. Some species require similar food and live under similar conditions of existence; they meet in competition and fight with one another; and, if they did not do so, the food-supply of a given area would be inadequate to support the offspring of all the pairs inhabiting that area. Generally speaking, the more closely related the forms happen to be, the more severe the competition tends to become; and it may be argued that in such cases a similar song would contribute to more effective distribution and in some measure provide against the necessity of physical encounter; that, in fact, it would stand in like relation to the success of all "the individuals concerned, as does the song to the individuals of the same species. But we must bear in mind that the primary purpose of song is to direct the females to those males that are in a position to breed; and to risk the possibility of prompt recognition in order that the males of closely related species should fight the less, would be to sacrifice that which is indispensable for a more remote and less important advantage.