For more than half a century the Fish and Wildlife Service and its predecessor, the Biological Survey, have been collecting data on the interesting and important phenomenon of the migration of North American birds. The field men of the Service have gathered information concerning the distribution and seasonal movements of the different species in many extended areas, from the Arctic coast south to the pampas of Argentina. Supplementing these investigations is the work of hundreds of volunteer ornithologists and bird students throughout the United States and Canada, who each year, spring and fall, forward to the Service reports on the migrations as observed in their respective localities. Added to the mass of data thus assembled is a rapidly growing recovery file of marked individuals. These data, together with other carded records gleaned by the Fish and Wildlife Service from a vast literature, constitute a series of files that now contain well over 3,000,000 entries, easily the greatest existing accumulation of information pertaining to the distribution and movements of North American birds. Not only do the facts thus assembled form the basis of regulatory action for the protection of the birds, but they also make it possible to publish scientific accounts of the ranges and migrations of the different species. They furnish the basis of this bulletin. The several important bird-protective measures adopted by State and Federal Governments, particularly those having as their objectives the conservation of the migratory song, insectivorous, and game species, can be effective only if they have intelligent public support. To increase such support, information must be more generally available on that little understood but universally fascinating subject of bird migration. A brief presentation of facts on the migratory habits of the birds scientifically gathered by the Fish and Wildlife Service over many years, will be helpful to bird-study classes, to conservation organizations, and to farmers and others individually interested in the welfare of the birds. In addition to his original investigations in the field and in the files of the Fish and Wildlife Service, the author has made free use of the writings of many other students of the subject. To all of these grateful acknowledgment is made. _The Mystery of Migration_