Where & why do raptors migrate?
Raptors usually migrate across a broad front over level terrain. High mountain ranges, oceans, and lakes are obstacles for migrants, creating leading lines and concentrating birds along narrow pathways. In North America, where the major mountain chains run north-south, cross winds hit the ridges and create updrafts favorable for slope soaring. Hawk Mountain is on the Appalachian Flyway.
For South American destinations, the major route for soaring species is through Mexico and Central America. More than a million turkey vultures, and broad-winged hawks have been counted at Veracruz, Mexico, where two barriers—the Sierra Madre mountains on the west, and the Atlantic Ocean on the east funnel migrants through a narrow corridor. Scarcity of food is the primary reason that most North American hawks leave their breeding territories in winter. Why raptors return in the spring is not so obvious. Ecologists believe lower predator and parasite populations, longer days for hunting, and most of all, more abundant food supplies increase breeding success in the temperate zone.
In some species of raptors, every individual migrates. In other species, only part of the population migrates and some individuals remain on the breeding grounds. Other species are completely sedentary. Overall, about 45 percent of all raptor populations migrate.
The distance of migration varies among species. Hawk Mountain’s bald eagles, for example, travel only as far as Florida. Other species like the broad-winged hawk, osprey, and the peregrine falcon, withdraw almost completely from the United States and Canada, with some individuals traveling thousands of miles into Central and South America.