MARSH WARBLER 



Marsh Warbler to prohibit the development of an earlier, and 

 probably more advantageous, habit of nest building. Such 

 a development may, indeed, be gradually taking place ; for if 

 at some earlier period the nesting instinct of this bird was 

 similar to that of the Eeed Warbler, we have an explanation 

 of this curious variation. We can account for the com- 

 paratively late arrival of the Eeed Warblers. They, in the 

 majority of cases, construct their nests upon reeds which only 

 attain the necessary height late in the season. Some, it is 

 true, make use of the old reeds only, but not early in the 

 season, since any tendency in the direction of too early nesting 

 would be held in check by the fact that such nests could not 

 fail to be more conspicuous, and therefore more liable to 

 destruction. The habit of late arrival has, in their case, 

 simply conformed to the needs of their environment. The 

 Marsh Warblers on the other hand no longer depend for the 

 construction of their nests upon the growth of any particular 

 plant ; they are no longer subject to the same selective agency, 

 and consequently we find the instinct of migrating at a more 

 or less definite period subject to considerable variation. To 

 them earlier nesting may now be an advantage, since the 

 likelihood of two broods being reared in one season is greater, 

 and so the tendency towards an earlier arrival may be receiving 

 encouragement instead of being checked; hence the extreme 

 variation. If the individuals which breed in England and the 

 western parts of Europe gain any advantage by their late 

 arrival, how can we explain the fact that those which inhabit 

 other parts of Europe, where the conditions of existence are 

 very similar, arrive so very much earlier ? In contrast with 

 this remarkable variation we have the relative constancy in 

 the date of arrival of the Eeed Warbler throughout western 

 Europe. Thus England is reached at the end of April or 

 the first week in May, France, Hungary, Germany and 

 Switzerland about the middle of April, Denmark at the com- 

 mencement of May, while Heligoland is passed on migration 

 in May. So that the arrival of this species in the various 



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