To return to what may be seen in nestling birds, n very young, and before their education has begun, if quietly approached and touched, they open their bills and take food as readily from a man as from the parent bird. But if while being thus fed the parent returns and emits the warning note, they instantly cease their hunger-cries, close their gaping mouths, and crouch down frightened in the nest. This fear caused by the parent bird's warning note begins to manifest itself even before the young are hatched--and my observations on this point refer to several species in three widely separated orders. When the little prisoner is hammering at its shell, and uttering its feeble _peep,_ as if begging to be let out, if the warning note is uttered, even at a considerable distance, the strokes and complaining instantly cease, and the chick will then remain quiescent in the shell for a long time, or until the parent, by a changed note, conveys to it an intimation that the danger is over. Another proof that the nestling has absolutely no instinctive knowledge of particular enemies, but is taught to fear them by the parents, is to be found in the striking contrast between the habits of parasitical and genuine young in the nest, and after they have left it, while still unable to find their own food. I have had no opportunities of observing the habits of the young cuckoo in England with regard to this point, and do not know whether other observers have paid any attention to the matter or not, but I am very familiar with the manners of the parasitical starling or cow-bird of South America. The warning cries of the foster parent have no effect on the young cow-bird at any time. Until they are able to fly they will readily devour worms from the hand of a man, even when the old birds are hovering close by and screaming their danger notes, and while their own young, if the parasite has allowed any to survive in the nest, are crouching down in the greatest fear. After the cow-bird has left the nest it is still stupidly tame, and more than once I have seen one carried off from its elevated perch by a milvago hawk, when, if it had understood the warning cry of the foster parent, it would have dropped down into the bush or grass and escaped. But as soon as the young cow-birds are able to shift for themselves, and begin to associate with their own kind, their habits change, and they become suspicious and wild like other birds.