Page:Popular Science Monthly Volume 7.djvu/203

Rh birds, the more prolonged autumn stay of the latter, we think, explains itself. In the spring, there is an object ever in view, while on their journey north—in autumn, their sole care is to be home in time; not so much to escape the coming cold, as to avoid being pinched by hunger.

We have seen that the first frost that but little affects vegetation does materially decrease insect-life; the swallows even anticipate this first frost, and, gathering in immense flocks, wing their way southward long before it comes. We can clearly see that the weather greatly influences, indeed governs, the migratory movements, in autumn, of the insect-eaters. It bids them depart, and, in general, they heed the bidding; but long after this, while there are yet berries, seeds, and fruits, to be obtained, the migratory vegetarians linger, in varying numbers, by the way.

Let us now glance at the abundant and well-known purple grakle or crow-blackbird (Quiscalus purpureus). The numbers of this (with us) partially migratory species that remain throughout the winter, as compared with those which are here during the spring and summer months, are about as three to one hundred, as near as we can judge; and, in proportion as the winter is mild, the percentage of those that remain is increased. In Massachusetts, this bird is strictly migratory; the great bulk of those that depart from the north, and from New Jersey, wintering in the Carolinas and Georgia. In this species, therefore, we have an example of a migratory bird that is gradually becoming more and more accustomed, not to the rigors of winter which birds are better able to withstand than they are supposed to be, but to the methods of our winter residents, such as woodpeckers, jays, and titmice, in procuring such food as can then be procured. Food, as a matter of course, and an abundance of it, must necessarily be obtained, and, on examination of the stomachs of grakles killed in January, we have found them filled with a half-digested mass of what appeared to be both animal and vegetable matter. If the grakles that remain during the winter are of a hardier constitution than those that migrate, then, as they mate very early in the year, and before the great bulk of the southern sojourners reach us, their offspring will naturally inherit equally vigorous constitutions, and, like their parents, will be more disposed to remain; at least a large proportion of them will be, and in this way, wholly through natural selection, a race of grakles, otherwise undistinguishable from the whole number of this species, will be evolved, that in time will wholly replace(?) the now migratory and semi-migratory individuals. If we have now correctly explained a change now in progress, in the habits of this and other species, then can we not, from it, gain a clew to one, at least, of the original causes of the habit of migrating?

But this we will discuss in the concluding part of our essay.

The act of migrating being the passage from one distant point to