Page:Popular Science Monthly Volume 76.djvu/547

Rh the watch, until suddenly the machinery would begin to work, and the long body of the larva would slide down the throat of the nestling, as if greased. The insect was laid crosswise between the mandibles, which closed upon it, but not always with sufficient force to hold it, for when the parent relaxed her grip, on more than one occasion it fell on the nest, and was picked up and offered again in the same manner. During this period of suspense, the mouth of the little bird would water copiously, and now and then the insect would be moved slightly by the parent, or withdrawn and returned to the same bird.

When the feeding is over, inspection (e) follows with clock-like regularity, provided always that behavior is free, and with head inclined to be prone, the parent inspects young and nest, with a view of cleaning them (/) which means the removal of the excreta, or of any particles of food which may have escaped the young. The cleaning instinct is very wide-spread among the whole class of birds, which from the human standpoint are probably the cleanest of all vertebrates which live out of water.

The study of the cleaning instinct in birds offers many surprises, and shows us plainly that besides the question of sanitation, which might be assumed to be of paramount importance, there is the element of concealment, which in the smaller and more timid species really counts for more, while of lesser significance is the value of the excreta as food for the adult. A young bird ordinarily mutes shortly after the food taken reaches its stomach, or at least after it is swallowed, and in so doing instinctively turns so that the raised hinder end of its body is directed toward the margin of the nest. Consequently the sac, when allowed to fall, usually lands on the nest-rim, when it does not reach the ground. The excreta in cuckoos, and in most passerine birds, to mention but two prominent orders, are in the form of tenacious, mucous sacs, which are snapped up as they leave the cloaca of the nestling, and are either eaten or removed (Fig. 14). This sac resembles a rubber water bottle with thin transparent walls closed on all sides, which can be rolled or picked up without soiling bill or fingers. Digestion is very rapid in nestlings, and remains of insects have been found in a sac from a black-billed cuckoo but four hours old. But while the digestive process is rapid, it is often very imperfect, and compact fruits like the blueberry will sometimes pass the alimentary tract without change. This in part explains the use of the excreta as food, and suggests that whether they are to be eaten or removed it is only a question of hunger at the moment. Robins, when they do not devour the sacs outright, carry them away, flying low with depressed head, and drop them a few rods from the nest, but are sometimes seen to peck at them after reaching the ground. In some species, like cedar-birds, and again in certain individuals only, the excreta are more regularly and