Page:The British Warblers A History with Problems of Their Lives - 5 of 9.djvu/75

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whether they are able to distinguish the call-note of their own from that of other young, it is difficult to say. The same factor which we discussed earlier no doubt guides them in regard to feeding in rotation; those that are the more hungry are possibly the more persistent in uttering their call-note and do so the more loudly. Owing to the conditions under which they are born and reared it is probable that there is a very considerable mortality amongst them, any tendency to weakness being rigorously eliminated. The young of other species of the same genus are comparatively secure even upon the ground, and can, indeed, often be found after they have left the nest hidden amongst the undergrowth. But the young Reed Warblers must be prepared to take their place in the struggle for existence immediately they leave the nest; any weakness that would prevent them from indefinitely clinging to the reeds until capable of considerable powers of flight would be fatal and could only result in their destruction; and to meet this difficulty it probably is the case that they remain in the nest longer than those of some other closely allied species, since any tendency to leave it too early would be held in check. The weakling would neither be able to withstand the cold, wet weather, nor could it keep up with its parents or even attract their attention when desirous of food. In fact, the struggle for existence must, in their case, be exceedingly severe.

Throughout the whole of June and July periodical outbursts of excitement occur amongst the different individuals inhabiting the reed bed, such outbursts being apparently not confined to one sex only, although in most instances the males are probably the sole actors. To determine what are the causes which lead up to these activities is difficult; it is not even possible to decide whether the participators are in earnest and the struggles genuine, or whether their behaviour is an expression of exuberance of energy. Play forms an important part in the life-history of many mammals. The rabbit, the hare, or the fox can be seen practising for the