Page:The British Warblers A History with Problems of Their Lives - 7 of 9.djvu/79

 fæces. A certain quantity of food may be necessary to set the machinery at work, to stimulate, that is to say, the nerves which control defecation; and this quantity may only be supplied to the first of the two offspring, and the second, though partially satisfied, may neither receive sufficient to bring about a similar result, nor to allay the hunger to such an extent as would make it incapable of the necessary effort to obtain the major share at the next opportunity. The whole system has probably been built up to bring about a certain definite result, and that result is the cleanliness of the nest. I have already called attention to the importance of cleanliness in this respect, and have pointed out that young birds could scarcely thrive in an insanitary nest. The perfection of the system can be better understood if we imagine the present regular method abolished. The young would then be fed accidentally, defæcation would occur at no specified time, the faces would be overlooked, and the nest consequently contaminated. Perfect as the system is, even now it does not always ensure complete cleanliness, for the fæces are occasionally allowed to remain in the nest. Experiments made with leaves placed in the nest bring about results very similar to those already referred to in the life of the Reed Warbler. A leaf is picked up by the parent and held for a few moments; it is then dropped, but again picked up and perhaps carried away or again dropped. Even when the faces are there awaiting removal a leaf may be carried away in place of them. However, it is well to state that considerable hesitation is displayed in their attitude towards such an obstruction; no doubt the interruption of the instinctive procedure is sufficient to account for their bewilderment.

The young are completely naked at birth, no down being visible, and the eyelids are sealed. For the first two days or so no very great change in their growth is perceptible, but on the third day the primaries become visible. Between the third and fourth days a considerable change takes place, the feathers sprouting on all the tracts, and the primaries being