This hawk, which breeds in open habitat throughout much of western North America, spends more than a third of its life migrating. Each year, all Swainson's Hawks (except for a few birds in central California) travel to their non-breeding home in southern South America. Like Broad-winged Hawks, Swainson's Hawks don't feed during this entire period. They tend to move in flocks following weather patterns that facilitate energy conservation: First, the birds circle high on rising warm air currents. As the air cools at higher altitudes, they glide in a straight line while gradually losing elevation. When another rising thermal is detected, they again circle up on the warm updraft.
Large flocks are not regularly recorded at most watch sites other than Corpus Christi, Texas, though a few hundred birds are seen each year at several points in the Southwest. The funneling effect of this species is most extreme in the area of the Panama Canal, where the entire population of Swainson's Hawks is believed to pass over Ancon Hill twice each year. Because of its tendency to feed on grasshoppers and other large insects, the Swainson's Hawk was heavily affected by pesticides, particularly in the part of Argentina where it spends the North American winter. As the hawks recover from their population decline, they're expected to begin appearing more regularly in greater numbers at U.S. hawkwatch sites.