Each spring, vast numbers of landbirds migrate northward across the Gulf of Mexico en route to breeding habitats in the United States and Canada from their wintering quarters in the neotropics. Following a short breeding season in the north, most of these birds return southward across the Gulf, their numbers then augmented by offspring produced over the summer. From the standpoint of sheer numbers, these annual trans-Gulf migrations constitute one of the great wildlife events of the world. Nevertheless, many basic aspects of the migrations have remained poorly known until now; indeed, until the 1950s, the very idea that birds routinely and "intentionally" migrate across the Gulf was treated with skepticism by some respected scientists