Page:The British Warblers A History with Problems of Their Lives - 8 of 9.djvu/71

 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 Reed Warbler, we have an explanation of this curious variation. We can account for the comparatively late arrival of the Reed 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 Reed 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 commencement of May, while Heligoland is passed on migration in May. So that the arrival of this species in the various