Page:Popular Science Monthly Volume 30.djvu/630

608 in May and leaving again in August. The former bird is large and strong enough to capture other grown birds and mice, and can thus always find a bounteous table spread for itself, while the others, confined by their more limited powers to smaller creatures, have to go away when, with the end of summer, these fail. The carrion-crow, to which everything that it is only possible to swallow furnishes a delicious feast, is not troubled by the severity of winter; but the rook, whose bill of fare is more limited, is in no condition to endure the scarcity of the winter months, and, therefore, on the approach of fall, it flies in large flocks over the Alps and Pyrenees, to the luxuriant fields of Southern Europe and Africa.

While this brief glance at the connection between the yearly migrations of birds and their food-supply must suffice for that point, the consideration of the distribution and accidental wanderings of these remarkable creatures, so far as it is related to their commissariat, demands more attention. It is evident that an animal species can live and thrive only when its accustomed provision, or some substitute similar in character to it, is present; where this is wanting, it can not gain a permanent footing; or it must adapt itself to its new relations, undergo gradual modifications in its habits and organization, and thus in the course of time become another and new variety. But wherever it can find its accustomed subsistence in the usual quantity and season, it will readily make itself at home without having to undergo any modification, unless it is provoked by other causes. If we ask what creatures—what birds, in the present case—have under particular circumstances the greatest chance for a wide distribution, the answer will be that they are those which, like the crow, the thrush, and the true shrike, are least particular in the choice of what they eat; and next, those whose appropriate food is most widely diffused. But the more closely a bird is adapted to particular kinds of food, and the more limited the circle of its distribution, the narrower will be the field of its residence. This is the case, not only with species, but in a wider sense with genera and families, and it is not to be overlooked that very widely distributed species are also frequently permanent residents. The most uniformly distributed animals under all conditions of climate and season are vertebrates, especially fishes and the smaller land-mammalia, and also some insects; and they also exhibit less important and diversified variations, and demand less on the part of their pursuers, than insects. Next to these are certain invertebrates of fresh and salt water; and among these the shore-species, particularly within their seasonal limits, display a great uniformity in all parts of the earth. Birds of prey, except the carrion-eaters, live on land-vertebrates. Three of their four families include quite or nearly cosmopolitan genera, and, what is rare among land-birds, only three species are wanting in very small districts. Fish and water animals constitute, with few exceptions, the food of the large groups known as the waders