Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/90

 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 condition 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 Reed 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 variation in different countries. For the late arrival of the Reed 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 disadvantages 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