Page:The British Warblers A History with Problems of Their Lives - 9 of 9.djvu/48

 This in outline is what seems to be the normal routine. There are plenty of deviations from the rule and plenty of variations of procedure; the same bird may be fed by both parents in quick succession, or by one parent again and again until well-nigh exhausted by the abundance of food showered upon it.

Numerous references have been made to the method which has been evolved for ensuring the complete removal of the excrement and thereby the cleanliness of the nest. The importance of cleanliness is alluded to in the life of the Whitethroat and it is unnecessary to discuss the matter further; attention may nevertheless be directed to the method by which this end is secured.

We must remember that cleanliness is only brought about by correlated behaviour on the part of parent and offspring. The usual procedure is as follows: the parent delivers food to one of the nestlings and then waits for it to eject the fæces; if it fails to do so the parent sometimes touches the anus with its bill, which seems to have the effect of stimulating the nerves which control defæcation; the fæces are then carried away and dropped some distance from the nest or swallowed. Yet this correlated behaviour on the part of parent and offspring, wonderful though it is, would not be sufficient to ensure complete cleanliness without the contributory physiological factor, i.e., the membranous sac or envelope in which the fæces are enclosed and which facilitates the act of removal. But is ease of removal alone sufficient to account for this wonderful contrivance? Ease of removal implies economy in time, and time is everything. That the young in the early stages of growth should sleep as much as possible, should be kept as warm as possible, and should run no risk of prolonged exposure is imperative if the species is to maintain its position. This biological end is furthered by the specialized activities of the parents, whose behaviour shows how admirably racial preparation has fitted them for the task. All their movements, or all her movements as the case may be, are delicately adapted to contribute to the main parental function of supplying