INSTINCT AND INTELLIGENCE IN BIRDS 543 



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 witli 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 copi- 

 ously, 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 parent, or withdrawn and returned to the same bird.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 reg- 

 ularity, 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 di- 

 gestive 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 sug- 

 gests 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 continu- 



