The fall migration of the Broad-winged Hawk is perhaps the best-known and most spectacular aspect of its ecology. Departing the breeding grounds ahead of most other raptors, perhaps forced to do so by scarcity of amphibian prey, it forms large flocks known as "kettles" that often number in the tens of thousands and include other raptor species. Often using thermals, most Broad-wings pass over a given area within a two-week period, though the first and last birds may be two months apart. Fall migration routes are generally somewhat east of spring routes. Most of the population passes along the Great Lakes, Appalachians, and Mississippi Valley, with smaller numbers following western mountain ranges and the Pacific coast. Long-distance water crossings are avoided. Flyways converge in south Texas, then follow the Gulf of Mexico coastline south. The winter range extends from southern Mexico to northern Argentina, with a small population wintering in south Florida and resident populations in the West Indies, where five of the six subspecies occur. Spring migration is more solitary, with few large concentrations reported outside south Texas and the southern shores of the Great Lakes; and is later than that of most raptors, again probably reflecting availability of amphibian prey.