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becomes of our summer birds? Where do they spend the winter? By what routes do they travel to their destinations? How do they find their way? For many centuries these and similar questions have puzzled the brain of man. In default of exact knowledge, fanciful theories have been advanced, such as that swallows hibernate in the mud, and that small birds cross the Mediterranean as passengers on the backs of cranes. Such notions have held their own well into modern times. Scarcely a hundred years have elapsed since systematic knowledge on the subject began to accumulate, and only in the last half century has there resulted any noteworthy progress toward a solution of the questions of migration.

For nearly twenty years the Biological Survey bas been accumulating data on the migration of birds. Its own field naturalists, whose visits have extended over the North American Continent from Guatemala to the Arctic Circle, have furnished voluminous notes, besides which the assistance of ornithologists throughout the country has been enlisted, so that reports are received in the spring and fall of each year from hundreds of observers. These reports give, for each species, the date when the bird was first seen, when it became common, and when it disappeared. Light-house keepers also have supplied valuable information concerning the destruction of birds at their lights. The facts thus gathered