WILLOW WARBLER 



there they will be found for the first two hours or so of day- 

 light singing incessantly, but for the remainder of the day 

 they behave as they do during the winter months, roaming 

 about in -flocks; it is not until some weeks later that the 

 females become sexually developed and commence to; seek for 

 mates. Territory is an essential to reproduction. By some 

 means a male must proclaim the fact that he is in possession 

 of a territory, and by some means the female must be made 

 aware of the presence of a male fit to reproduce. I see no 

 difficulty therefore in believing that all the particular sounds 

 emitted at this season are thus of some use. And since, on 

 the average, the stronger males only will be capable of obtain- 

 ing a territory and thereby attaining to reproduction, may it 

 not in justice be argued that a gradual increase in their power 

 of producing sounds will automatically result ? It is difficult to 

 obtain any concrete evidence in proof of this. I will mention 

 a few facts which may be used in support of it, and there leave 

 it. The males we hear in full song in the spring are those 

 which own territories and ultimately reproduce ; I can recall 

 no instance of a wandering male, that is to say, a male without 

 a territory, in full song in the height of the breeding season. 

 Males in search of territories sometimes pass through those 

 already occupied and, as far as I have been able to 

 observe, they remain silent during their transit. In the case 

 of many species the song is uttered less frequently upon the 

 arrival of the females and in some instances practically ceases. 

 When two males occupy adjoining territories, and one of them 

 utters its song whilst on the boundary, the other not infre- 

 quently attacks it forthwith. "■/„.._;:..'. :;.: :;:?'..::■ 



The migration of the females extends over some -days. 

 The first females arrive about a week or ten days; after the 

 first males, but this interval is subject to variation according 

 to the seasons. Thus it happens that males with adjoining 

 territories- do not necessarily all obtain a- female on the. same 

 day. The difference is sometimes considerable, and may even 

 amount to as much as a week. This discrepancy in the time 



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