SEASONAL CHANGE OF RELATIONSHIP 229 



we start on firmer ground, because the one 

 factor which introduced an clement of uncer- 

 tainty the female can be definitely excluded ; 

 at least it seems so to me, for grunting even 

 that her presence is the condition under which 

 the pugnacious nature of the male is rendered 

 susceptible to stimulation, it is difficult to see 

 why a male of a different species should supply 

 that stimulus, or what biological purpose could 

 be served by its doing so. 



When dealing with the attitude of a male 

 towards others of its kind, we attached consider- 

 able significance to the fact that its pugnacious 

 nature gained or lost susceptibility according 

 to the position which it happened to occupy. 

 We found, it will be remembered, that the 

 same bird that was pugnacious in its own 

 territory took no further interest in its opponent 

 when the boundary was passed ; and, moreover, 

 that if it happened to wander into an adjoining 

 one, it made no real effort to defend itself 

 when attacked, but returned forthwith to its 

 own headquarters. It remains to be shown 

 whether the rivalry between different kinds of 

 birds is similarly related to the position which 

 the opponents happen to occupy at the time. 



First, then, there is the general consideration, 

 namely, that the enmity occurs for the most 

 part just at the time when the territories are 

 in process of being established. During autumn 

 and winter, many birds of more or less close 

 affinity assemble together in flocks, wherever 

 the supply of food is abundant, and are then 



