Page:Popular Science Monthly Volume 62.djvu/117

Rh possible, however, that protective coloration was preceded by the development of precocious powers of flight, which have since been discarded by all save the game-birds. Probably this rejection was brought about because excessive activity on the ground was found to be as fatal as in the trees; since the young, in escaping from one danger, would be liable to run into another, or to stray too far away to render return possible.

At the present day, though the young of all the game-birds are protectively colored they have yet preserved more or less perfectly their earlier precocial powers of flight, the birds, when escaping danger, using their wings either like the ostrich, as an aid in running, or in actual flight, and there is evidence to show that the broods in consequence suffer. As an example, the observations of Mr. Ogilvie Grant on this subject may be cited. In writing of the common pheasant, he tells us that the mother, on alarm, with a warning note to the young, at once flies off and leaves them to take care of themselves. This they do by scattering in all directions, and then squatting down and trusting to their protective coloration for safety. Quiet restored, the parent returns, often only to recover but three or four of her chicks, the rest having strayed to such a distance that they are left to perish.

Thus, then, the hypothesis of precocious flight seems by no means an improbable one. Its development will be easy to understand when it is remembered that the raw material therefor is furnished by that aberrant member of the game-birds—the hoatzin. The life-history of this bird will be discussed later.

We may pass now to a consideration of those species which, retaining their arboreal nesting habits, have adopted the method of curtailing the activity of the young. This process was accomplished by reducing the food-yolk within the egg, and thus inducing an earlier hatching period. We may approximately measure the extent to which this reduction has been carried by the degree of helplessness displayed by the newly hatched bird, and by the nature and extent of its clothing.

The number of species which have adopted this expedient outnumber those which have not, and this speaks volumes for its success. As examples, we may instance the passerine or song-birds, parrots, cuckoos, birds of prey, cormorants and their allies, and the storktribe. The young of these are all born extremely helpless, many perfectly naked, others enveloped in a thick coat of down, whilst in some down is developed soon after hatching, and, in a few, not at all.

Having once however reduced the amount of food-yolk, return to the older fashion of nidifugous young became impossible, and this explains why nidicolous young are still born to those parents which have adopted the practice of depositing their eggs upon the ground. It proves that the arboreal habit has been forsaken since the