WARBLERS 223 



the circumstances in which they are placed. If 

 the territory of a Thrush is invaded the Thrush 

 is the aggressor, and, conversely, if that of the 

 Blackbird is threatened, the Blackbird becomes 

 the aggressor ; and so, when the territories of the 

 two birds are adjacent or overlap, as frequently 

 they do, there is constant friction, resulting in 

 quarrels which attract attention on account of 

 the noisiness of the birds. 



All the Warblers are exceedingly pugnacious, 

 the fighting being especially severe between 

 those that are very closely related. The Black- 

 cap and the Garden-Warbler are constant rivals, 

 and the scenes which can be witnessed when 

 the two meet in competition are interesting 

 from many points of view. The birds not only 

 pursue and fight with one another, but their 

 emotional behaviour reaches a high level of 

 intensity — excitable outbursts of song are in- 

 dulged in, tails are outspread, wings are slowly 

 flapped, and feathers raised — in fact the attitudes 

 assumed are similar in all respects to those 

 which occur during the contests which are so 

 frequent between the respective individuals of 

 each species ; and it would be difiicult to point 

 to any one item of behaviour which is not 

 also manifest at one time or another during the 

 battles between these rivals, and still more 

 difficult to trace any difference iii the intensity 

 of the excitement. And if we are satisfied that 

 the fighting in the one case is purposive, so, too, 

 must we regard it as having some biological 

 purpose to serve in the other. But the Garden- 



