Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/80

 are complete in themselves, and both birds are equally good imitators of the songs of other species. Between these two extremes there are a large number of notes uttered by different species which are very similar, and may or may not be imitations. I refer more especially to the call-notes, which, being as a rule single notes, would the more easily be imitated, and at the same time the more likely to be a natural production. But when we consider the extraordinary number of songs, call-notes, and combinations of sounds, produced by bird life in general, we ought surely to be astonished, not that we can occasionally detect a similar note in two different species, but that we cannot more frequently do so.

In the song of different individuals there is little variation, nothing in fact to show that it reaches a higher degree of perfection year by year, excepting perhaps that a greater number of imitations may be remembered and reproduced.

Their food consists principally of insects, Chironomidæ and Limnobiidæ being devoured in large quantities. But Aphides may be said to form their staple diet, and these they find clustering on the leaves of the reeds and on the branches, twigs, and buds of the alders and willows surrounding the reed bed. It is always a difficulty to determine exactly what the insects are which are being taken and supplied to the young, since their digestive powers act so rapidly. Even dissection of a young bird immediately after the parent has supplied it with food is of little use, the insects by that time being reduced to a pulp by the digestive fluid, and consequently beyond recognition. This result is not due to their being supplied with regurgitated food, for one can see the insects being caught, carried, and delivered even in a living condition into the throats of the young. For the purpose of identifying the insects, I have from time to time made a number of experiments, but hitherto with little success. Small pieces of cotton-wool placed in the throat and secured with a fine thread prevented some of them from being swallowed, if removed immediately the