The migration of birds had its beginning in times so remote its origins have been largely obscured and can be interpreted now only in terms of present conditions. The causes underlying migration are exceedingly complex. The mystery that formerly cloaked the periodic travels of birds, however, has been largely dispelled through the fairly complete information now available concerning the extent and times of seasonal journeys of most species. Many gaps still remain in our knowledge of the subject, but present knowledge is being placed on record, and the answers to many uncertainties that continue to make bird migration one of the most fascinating subjects in the science of ornithology must be left for future studies. In some areas we are on the threshold of discovery. More and more sophisticated approaches including radar, radio telemetry, computer processing of banding data, and physiological and behavior studies are being developed. With the widespread use of these new techniques, we are beginning to realize the benefits, aside from aesthetic reasons, for studying migration. Radar alone has aided tremendously in documenting flock size, heights, and speeds of migration as well as the descriptions and locations of patterns and routes of specific migrants in relation to aircraft flight lanes. Recent studies have indicated local, nonmigratory populations of various blackbirds cause nearly all of the rice damage in southern States and the "hordes from the North" contribute very little to the losses. In addition, the transport of arborviruses from one continent to another via these long distance migrants is being investigated. People have started to uncover the secrets of migration and utilize this knowledge for the betterment of our society. Each kind of bird seems to have its own reaction to the environment, so that the character of movement differs widely in the various species, and seldom do any two present the same picture. In fact, bird migration has been described as a phase of geographic distribution wherein there is a more or less regular seasonal shifting of the avian population caused by the same factors that determine the ranges of the sedentary species. If this view is correct, then it must be recognized that the far-reaching works of man in altering the natural condition of the Earth's surface can so change the environment necessary for the well-being of the birds as to bring about changes in their yearly travels. The nature and extent of the changes wrought by man on the North American Continent are readily apparent. Extensive forests have been burned or cut away, rolling prairies turned over with the plow, and wetlands drained or filled. Their places have been taken by a variety of human activities. These great changes are exerting pressure on native bird populations, and various species may be either benefited or adversely affected.