Now it is that part of the behaviour routine which has reference to the relationship between one bird and another upon which, for the time being. I wish to dwell; for the interest that A displays in B is by no means always the same—it changes according to the season, and this change can be observed to be uniform throughout a wide range of species.
In winter, in whatsoever direction we turn, we observe not only that different individuals but that different species also collect together in flocks. And since food at that season is not always easy to obtain, and, moreover, is only to be found in certain situations, which are limited both in number and extent, it would seem that such assemblages are in the main determined by accident. No doubt the abundance or the scarcity of food does determine the movements of birds, and hence to that extent may be held to account for the flocks. But we shall but deceive ourselves if we think that it is the sole or even the principal reason, or that the situation is in no wise affected by internal factors. The behaviour of the individual in relation to the flock bears ample testimony to the presence of a gregarious impulse which derives satisfaction from the fact of close association.
As an illustration, let us take a bird whose movements are easily watched, and in whose hereditary constitution the impulse to which I allude seems to be strongly implanted—the Curlew. When the breeding season is over. Curlew leave the mountain and the moor and
