Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/78

 the departure of a parent from the nest and its arrival with a fresh supply of insects varies considerably. Sometimes it appears to be in no great hurry and may not return for twenty minutes or more, whilst at other times it seems to be equally anxious that there should be no delay, and consequently returns in from five to six minutes. The parents as a rule approach the nest separately, but occasionally they arrive together, and when this occurs the same offspring may be fed by both of them. Defæcation usually ensues a few moments after the young one has been fed, and the parent waits for this to happen, and then carries the fæces away or swallows them. The routine of feeding is very similar to that which occurs in the case of the Reed Warbler; the bird, that is to say, which is the more hungry, is the more vigorous in stretching out its neck, and the more liable consequently to receive food. We can therefore explain the behaviour of the parents at the nest without imputing to them any intelligent control. There still remains, however, one point in connection with the care of the offspring, which, though appearing to be purposeful, may nevertheless be one of those beautiful adjustments of Nature, the development of which can be referred to selection. It is this: defæcation usually ensues after food has been supplied by the parent, who then carries the fæces away and thus prevents the contamination of the nest. Now it often happens that the parent divides each supply of food that it brings between two of the young, and no doubt economy is thus effected in the time and energy which it is compelled to devote to securing the necessary food. But it is clear that if both the young were to eject the fæces forthwith the parent could not conveniently remove them and Nature's object would not thereby be attained. If, however, the methods adopted by the parents be closely observed, it will be noticed that the various insects are not divided equally between the two offspring, but that one receives a larger proportion than the other. It may well therefore be that there is some relation between the method thus adopted and the ejection of the