WILLOW WARBLER 



and consist of rapid pursuits, collisions in the air with con- 

 siderable clicking of bills, and the usual signs of excitement 

 common to the bird. Generally speaking, the conditions of 

 existence imposed by the law of territory may at any moment 

 become so severe as to necessitate the individuals of not only 

 the same species, but also of others closely allied, struggling 

 together for possession of a breeding home. In the case of 

 the Willow Warbler and Chiff-chaff we seem to have an 

 instance, and in truth it would be a difficult matter to discover 

 two species to which such an extension of the law of territory 

 might be more likely to apply. So closely do they resemble 

 one another in structure and colouring, that expert knowledge 

 is required to decide whether some particular skin belongs to 

 the one or the other ; and in life, where a close examination is 

 impossible, it is often difficult to arrive at a decision. The 

 resemblance, moreover, does not end with appearance ; they 

 live in a similar environment, require similar food, and exhibit 

 characteristics which have much in common. Is it not then 

 possible — is it not indeed highly probable that there is a con- 

 stant struggle between them, and that their future position, 

 the survival or decrease and possible disappearance of one or 

 the other, is gradually being worked out before our eyes ? 

 The case is an interesting one, but perhaps not more interest- 

 ing than many others if we only had sufficient facts to rely 

 upon. The Willow Warbler is the stouter and no doubt the 

 stronger of the two, and, as already mentioned, it seems more 

 often to be the aggressor. Physical strength alone, however, 

 does not decide survival ; activity and power to resist 

 unfavourable conditions are of equal value. The Willow 

 Warbler may be gaining so far as direct struggle resulting 

 from the law of territory is concerned, but, on the other hand, 

 we must bear in mind the ceaseless activity which is so 

 prominent a characteristic in the life of the Chiff-chaff, and 

 remember that activity counts for much when unfavourable 

 conditions arise. Scarcity of food, brought about by late 

 frosts or exceptionally cold weather accompanied by blizzards, 



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