This eastern species is the most gregarious of all of the migratory raptors. Flocks of several hundred thousand birds form each year and move in one or two waves down the eastern flyways before joining into one huge flock along the Gulf Coast of Texas. Counts at the hawkwatch site near Corpus Christi have approached the 1 million bird mark in several years.
One of the earliest species to initiate migration, Broad-winged Hawks leave the northern parts of their range in early August. The birds don't feed during migration, and the main factors influencing their movement through the entire eastern United States are weather patterns -- the largest movements of birds take place just after the passage of cold fronts. With little variation, the entire North American population of Broad-wing Hawks passes from the Great Lakes and Northeastern states in early September, and passes into Mexico by the first week of October. A similar migration takes place among American birders who flock to the Mexican city of Veracruz in the fall to witness the "River of Raptors" as it passes through a coastal bottleneck nearby. Visitors can see flocks of hundreds of thousands of Broad-winged Hawks and vultures mixed with up to seventeen raptor species.