Page:Popular Science Monthly Volume 62.djvu/118

112. Some, like the cormorants, herons and certain of the gull-tribe, for example, build as occasion demands, either on the ground or in trees. Now it is interesting to note that among these birds the young cormorants and herons are completely nidicolous, the young gulls only partially so, whilst the near and less specialized allies of the gulls, the plovers, have nidifugous young. This indicates that in the gulls the food-yolk is in process of reduction. To species breeding in large colonies, or on ledges of precipitous cliffs, the reduction of the food-yolk and helplessness of the young are obviously advantageous.

That this is so may be seen in the case of the colony-breeding species, since it would be impossible for the parents to recognize their own offspring, if nidifugous, when running about amid those of their neighbors. In consequence a large number would almost certainly go unfed and soon starve, whilst great activity among the young of the cliff-breeding species would be accompanied by an enormous mortality, owing to falls from the cliff.

It is contended that the facts so far submitted amply justify the interpretation put upon them, but the following instances should carry conviction. In the aberrant South American game-bird, the hoatzin, we have probably a direct survival of the protoavian type of nestlings. They are, of course, nidifugous, but they differ from all other nidifugous young in the prehensile character of their wings, which are armed with large claws borne upon the thumb and index digit. Claws on the wing are common among birds, and hitherto they have been regarded merely as vestiges—indices of a reptilian ancestry. The part which they play, however, in the life-history of the hoatzin, coupled with certain correlated modifications to be discussed presently, shows that they have a wider significance than this.

The adult hoatzin is an absolutely arboreal bird, inhabiting the dense scrub and trees bordering the lagoons and river banks of British Guiana and the Amazon Valley. Its powers of flight are extremely limited, and it has never been observed to alight upon the ground.

The young, like those of other nidifugous birds, are clothed in down, conspicuous, in the present instance, for its hair-like appearance and sparse distribution. But whilst the locomotion of the nidifugous young of other birds is bipedal that of the hoatzin must be described as quadrupedal, the wings as well as the legs being brought into requisition as the birds make their way along the branches. Even the beak is sometimes used, as in the parrots.

In a wing used as a prehensile organ we should expect to find certain peculiarities which would not be observable in the normal wing. These are not wanting. One of the first points which attract attention in the examination of such a wing is the great length of the hand, which