262 RELATION OF TERRITORY TO MIGRATION 



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 movem,ents 

 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 



