Page:Condor6(5).djvu/6

116 from these various sources form the largest amount of material on bird migration ever collected in this country, and permit broader and safer generalizations than have heretofore been possible.

For more than two thousand years the phenomena of bird migration have been noted; but while the extent and course of the routes traversed have of late become better known, no conclusive answer has been found to the question, why do birds migrate? Some dismiss the subject with the statement that fall migration is caused by failure of the food supply, spring migration by love of home. All are familiar with the rush of waterfowl northward so early that they are often forced by storms to retrace their flight; and all know that robins, bluebirds, and swallows, following closely in the rear, sometimes lose hundreds out of their flocks by cold and starvation. If strong home love causes these birds thus to hazard their lives, why do they desert their home at the earliest possible moment; and if fall migration is caused by lack of food, why does it commence when food is most abundant? Data recently collected at the Florida light-houses by the Biological Survey show that southward migration begins at least by the 10th, and probably by the 1st of July, insect-eating birds departing when their food supplies are most plentiful, and seed eaters just before the heyday of harvest.

The broad statement can be made that the beginnings of migration ages ago were intimately connected with periodic changes in the food supply, but this motive is at present so intermingled with others unknown, or but imperfectly known, that migration movements seem now to bear little relation to the abundance or absence of food.

How do birds find their way over the hundreds or thousands of miles between the winter and summer homes? Among day migrants sight is probably the principal guide, and it is noticeable that these seldom make the long single flight so common with night migrants. Sight undoubtedly plays a part in guiding the night journeys also; on clear nights, especially when the moon shines brightly, migrating birds fly high, and the ear can scarcely distinguish their faint twitterings; if clouds overspread the heavens, the passing flocks sink their course nearer to the earth, and their notes are much more distinctly heard; and on very dark nights one may even hear the flutter of vibrant wings but a few feet overhead. So far as known, birds never intentionally migrate above the clouds, and when suddenly formed vapor cuts them off from sight of the earth, they lower their flight until the friendly landscape is again visible. Nevertheless, something besides sight guides these travelers in the upper air. In Alaska a few years ago members of the Biological Survey on the Harriman expedition went by steamer from the island of Unalaska to Bogoslof Island, a distance of about sixty miles. A dense fog had shut out every object beyond a hundred yards. When the steamer was half way across, flocks of murres, returning to Bogoslof after long quests for food, began to break through the fog wall astern, fly parallel with the vessel, and disappear in the mists ahead. By chart and compass the ship was heading straight for the island; but its course was no more exact than that taken by the birds. The power which carried them unerringly home over the ocean wastes, whatever its nature, may be called a sense of direction. It is probable that this faculty is exercised during migration.

Reports from light-houses in southern Florida show that birds leave Cuba on