BRITISH WARBLERS 



which accompanies them, and in the degree of adaptation to 

 environment attained. The comparisons which we have made 

 are not then without meaning. They hear testimony to a con- 

 dition of instability in the behaviour of the Marsh Warbler 

 which leads us to believe in the possibility of some change 

 having recently been effected. We came to the conclusion, it 

 may be remembered, that the nest of the Eeed Warbler was a 

 beautiful illustration of adaptation to a special environment. 

 Its depth allowed the reeds to sway without jeopardising the 

 safety of the young, its shape allowed of its depth, and the 

 means by which it was carefully woven to the reeds made it 

 amply secure from all excessive movements. But upon 

 examining the nest of the Marsh Warbler we found that 

 constancy of type, which so often betokens adaptation, 

 replaced by fluctuating variability; we found examples of both 

 deep and shallow types, nests on the one hand carefully woven 

 to upright branches where the necessity for so doing was not 

 obvious, and on the other insecurely slung to the surrounding 

 branches; and just as the former entailed a needless waste 

 of energy, so the latter courted needless disaster for the 

 offspring. We found, in short, certain conditions prevailing 

 which admitted of but one interpretation, namely, that the 

 nesting instinct was not properly adjusted to the environment 

 now inhabited by the bird. And with regard to the period of 

 arrival we saw that this too was subject to remarkable varia- 

 tion in different countries. For the late arrival of the Eeed 

 Warbler there is a cause. It builds in a special situation, and 

 until the new reeds are of a certain height, the facilities 

 for building are not so great, nor is the same measure of 

 protection afforded ; consequently the date of arrival is 

 adjusted to the growth of the reeds. On the other hand the 

 Marsh Warbler gains no advantage by so late an arrival at its 

 breeding haunts, but rather exposes itself to all the disadvan- 

 tages inseparable from late nesting. For it the conditions are 

 equally favourable to reproduction a month earlier in the 

 season, which is shown by the fact that individuals do arrive 



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