Page:The British Warblers A History with Problems of Their Lives - 6 of 9.djvu/23

 and consist of rapid pursuits, collisions in the air with considerable clicking of bills, and the usual signs of excitement common to the bird. Generally speaking, the conditions of existence imposed by the law of territory may at any moment become so severe as to necessitate the individuals of not only the same species, but also of others closely allied, struggling together for possession of a breeding home. In the case of the Willow Warbler and Chiff-chaff we seem to have an instance, and in truth it would be a difficult matter to discover two species to which such an extension of the law of territory might be more likely to apply. So closely do they resemble one another in structure and colouring, that expert knowledge is required to decide whether some particular skin belongs to the one or the other; and in life, where a close examination is impossible, it is often difficult to arrive at a decision. The resemblance, moreover, does not end with appearance; they live in a similar environment, require similar food, and exhibit characteristics which have much in common. Is it not then possible—is it not indeed highly probable that there is a constant struggle between them, and that their future position, the survival or decrease and possible disappearance of one or the other, is gradually being worked out before our eyes? The case is an interesting one, but perhaps not more interesting than many others if we only had sufficient facts to rely upon. The Willow Warbler is the stouter and no doubt the stronger of the two, and, as already mentioned, it seems more often to be the aggressor. Physical strength alone, however, does not decide survival; activity and power to resist unfavourable conditions are of equal value. The Willow Warbler may be gaining so far as direct struggle resulting from the law of territory is concerned, but, on the other hand, we must bear in mind the ceaseless activity which is so prominent a characteristic in the life of the Chiff-chaff, and remember that activity counts for much when unfavourable conditions arise. Scarcity of food, brought about by late frosts or exceptionally cold weather accompanied by blizzards,