Page:The British Warblers A History with Problems of Their Lives - 8 of 9.djvu/66

 different, the least inquisitive must wish to know what biological significance, if any, can be attached to this fact. If it were not that these two birds happen to live under conditions which are identical in all respects, we should not, I suppose, question the serviceableness of a feather lining; we should dilate at length upon the advantage of warmth to the young when the parents are away in search of food, and probably affirm that a cold breeding season only was required to decimate an imaginary race that had not acquired this method of securing warmth for their offspring.

Six or seven eggs represent the full clutch of the species, though five are sometimes spoken of as the full complement. The number equals, therefore, that of the Willow Warbler, or Chiff-chaff, but slightly exceeds the average of the majority of Warblers. One egg is laid with fair regularity every twenty-four hours until the clutch is completed. I do not know whether the slight excess in the number of eggs laid by these three species bears any relation to the shape of the nest. A covered nest must, one would suppose, be of considerable advantage in preserving the young from exposure, and, by to some extent relieving the parents of the necessity for brooding, enable them to secure food for a larger number of young. The whole problem of the relation between the type of the nest, the number of eggs, and the behaviour of the parents is somewhat intricate. I am taking it for granted that there is some relation—the evidence seems to point in that direction—although it is possible to find plenty of exceptions to any rule we may formulate upon the subject. Incubation lasts approximately twelve days. At birth the young are naked except for a little down, and their eyelids are sealed. No spots are visible at the base of the tongue. When the young are hatched the female betrays considerable anxiety. Flitting from branch to branch she keeps uttering her plaintive note, approaches the nest closely, then retires, and hesitates thus for some time before finally entering. Of emotional behaviour when she is frightened off her nest, there seems to be little