230 WARFARE BETWEEN DIFFERENT SPECIES 



not only sociable, but, so there is reason to 

 believe, are mutually helpful both in discovering 

 the necessary means of subsistence which are 

 often none too plentiful, and in affording pro- 

 tection from enemies, which, on the contrary, 

 are often numerous. That the different units 

 of which these flocks are composed should live 

 on amicable terms is therefore as necessary 

 for the welfare of the whole community at 

 this particular season as that the different 

 individuals of the same species should do so. 

 But just as the sociable relations, which obtain 

 between these individuals throughout the winter, 

 undergo a marked change at the commencement 

 of the breeding season, so, too, do different 

 species, which habitually associate together, 

 suddenly become hostile to one another. This 

 change is coincident in time with the rise of 

 the organic condition which leads to the 

 establishment of territories ; and the hostility 

 continues, though in diminishing degree, 

 throughout the breeding season, and dies away 

 the following autumn. 



For example, different Warblers resort to 

 the elders {Sambucus nigra) in September, and 

 there pass much time feeding on the fruit which 

 is then ripe and often abundant. In the same 

 bush there may be Blackcaps, Garden -Warblers, 

 Whitethroats, and Lesser Whitethroats, some 

 preening their feathers, others searching for 

 the berries, others again, with feathers relaxed, 

 making feeble attempts to sing. Occasionally 

 there may be a scuffle, perhaps between a 



