Page:Popular Science Monthly Volume 77.djvu/99

Rh "anticipates" the nest. Every one who has given much attention to the activities of birds in the field must have found isolated eggs lying on the ground. Such prize packages are probably more common than we might be led to suppose, for they can not long exist wherever snakes, rodents and other prowling animals abound.

With most birds the act of prematurely dropping an egg can be only a sporadic or casual variation. Without doubt, in the course of time a proper nest is built; eggs are laid, and the normal cycle is rounded out to completion. It is quite possible, on the other hand, that all such eggs are not immediately neglected, but that they are sometimes carried away, and "concealed" by dropping them in another bird's nest, although we have no observation to support such a view directly. It is known, however, that certain birds, such as the blackbilled cuckoo, will upon disturbance remove its eggs from the old nest to a new one or to a place of safety. It is also certain that the premature egg is at times laid direct in another bird's nest, which the intruder will often strive to possess by force, and may even succeed. Thus, Davidson, who is quoted by Bendire, found a black-billed cuckoo and a mourning dove sitting on a robin's nest together. This nest was in reality double, and contained two eggs each, of the cuckoo and dove, and one of the robin. The cuckoo managed to get possession of the nest before the robin had finished her work, and filled it with rootlets, but the robin held its ground long enough to deposit an egg. The fact that the cuckoo had "filled it nearly full of rootlets" is a very interesting circumstance, for it shows how completely instinct held the reins of action. This robin's nest seems to have served as a site on which the cuckoo strove to erect one of its own. The dove, noted for its strong parental instincts, had evidently come last, and her eggs were the only ones in which incubation had not begun.

Such a case seems to present us, as in a picture, with one of the steps in the process through which the most remarkable of all the known instincts of birds, that of parasitism, has been brought about.

Certain cowbirds of the new world and cuckoos of the old steal the nests of other birds, but usually only long enough to deposit an egg of their own, which is left to its fate. If tolerated, as is apt to be the case, the stranger is hatched with the other eggs, and the owner of the nest assumes the role of nurse or foster-parent. If a cowbird, the foundling soon smothers the proper young, and if a cuckoo, it evicts them. The cuckoo seems to react to a contact stimulus of a disagreeable kind, and when from one to three days old, while still blind, it strives to get egg or nestling on its broad, depressed back, and