Page:The British Warblers A History with Problems of Their Lives - 4 of 9.djvu/39

 was this, that the leaf was often removed and dropped some distance away from the nest, but not infrequently swallowed. Both male and female acted precisely in the same way.

Here, then, by a happy chance, an incident occurs which seems to momentarily lift the veil so completely shrouding the subjective states of the lower animals. We can well understand how necessary it must be for the welfare of the offspring that the nest should be uncontaminated by the excrement, and in many cases Nature has provided means whereby this end can be attained. In those instances in which the nest is enclosed, or lightly and loosely built of dead vegetation, it is even more essential that the means should be in every way adequate; and we find, as a rule, that this is the case, for the fæces are enclosed in this membraneous sac, which the parents are thus enabled to remove in their entirety. But amongst other species, where the complete cleanliness of the nest is not of so great importance, the young are themselves able to eject the fæces over the side of the nest. We therefore see at work a series of activities which, combined, result in a perfect system of cleanliness; the parent bird brings food and delivers it into the gape of a young one, it then waits for the fæces to be ejected, and if this is not done administers a slight tap with its bill on the anus, which is sufficient to produce the desired result. It then carefully lifts the membraneous sac, and, flying away, drops it some distance from the nest.

There can be little doubt that the whole procedure is congenital, for how, otherwise, should a young one know how to respond to the touch of the parent's bill? or how could the parents know what, under the circumstances, was necessary? In neither case could it have been learnt by tradition.

The system is perfect, and we can recognise the need of removal, but it is not so easy to understand what useful purpose is served by the swallowing of the excrement; the removal