Migration Info The fall migration of this species is a spectacular phenomenon, as virtually all of the world's population of Swainson's Hawks funnels through Central America within a few days. Until very recently, these hawks were thought to disperse across the whole of South America, and no one really knew where or how they spent the northern winter. Not until 1995 did biologists discover the relatively small area in the pampas of Argentina where these hawks spend the winter feeding almost exclusively on grasshoppers. While studying a population of Swainson's Hawks in eastern Oregon, biologists noticed that there were years in which as many as 40 percent of the breeding birds were not returning in the spring. This led to the radio-tracking of a bird to the previously unknown wintering grounds in Argentina. What the biologists found there was shocking: The hawks were feeding on insects that had been treated with pesticides and were dying by the thousands. Conservationists appealed to the Argentines, who made several changes in their pest-control practices, which have reduced Swainson's Hawk mortality.