Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/47

 occasionally three, but the first one as a rule receives the larger share of food and ejects the fæces, which are thereupon swallowed by the parent or carried away and dropped some distance from the nest. Does the parent bird remember which of its young it fed at the last visit, or is the process in this respect a haphazard one? My interpretation is simple enough though not altogether free from the possibility of criticism; it relieves the parent of all responsibility but at the same time leaves nothing to chance. I regard the sensation of hunger as the guiding factor. According to the strength of this sensation the young bird is active in stretching up its neck and in struggling towards its parent, and on the average the more active individual is the one that secures the food. There is therefore no need for the parent to remember to which of its offspring it attended on the last occasion; its business is to place the food in the nearest open throat; the rest is done for it. But, it may be said, is not hesitation shown by the parents as they stand upon the side of the nest and contemplate their offspring before finally parting with the larvæ they are carrying? Certainly, I reply; but is not their behaviour related to that of their offspring? Do they not hesitate only because the young hesitate also as the time draws nigh for sleep and response is therefore weak? I think we can explain it thus: we must regard the parents and offspring not as separate units, but as a biologically determined whole within which the sequence of activities forms a complete circle. The young bird that is hungry is the more active in stretching up its neck and thus has a greater chance of securing food. Having received it, it proceeds to eject the fæces, which are carried away by the parent. Hunger being temporarily appeased, it lies in the bottom of the nest taking but little heed of the parent returning with a fresh supply, and though it may open its mouth, yet the persistent striving towards some end is manifestly absent; digestion however proceeds apace, the hunger sensation once more asserts itself, response becomes increasingly active, and in time it receives a further supply.