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 return to the coast or tidal estuaries for the remainder of the year. Here, at low water, they find an abundant supply of food--crustaceans amongst the sea-weed upon the rocks, and lobworms (_Arenicola piscatorum_) in the mud as the tide advances or recedes. But when the tide is full, they retire to those parts of the shore that remain uncovered--to isolated rocks, or to sand-dunes, or it may even be to pasture-land in the neighbourhood. During this period of repose large numbers of individuals gather together on a comparatively small space of ground. They are not constrained to do so by any shortage of accommodation, nor by any question relative to food, nor, for the matter of that, by any circumstance in the external environment; they are brought together solely, this at least is the impression that one gains, by some inherited impulse working towards that end. And their subsequent course of behaviour tends to confirm that impression. For if we watch the gathering together of the different units of which the flock is composed, and study more particularly the emotional manifestation which accompanies their arrival and departure, we shall find that the coming of a companion arouses some emotion which is expressed by a vocal outburst that sweeps through the flock.