Swainson’s Hawk is the most migratory North American hawk. It breeds commonly on the Great Plains, as well as in grasslands and agricultural areas in western deserts and the Central Valley of California. From September to November it migrates in large flocks south through Mexico and Central America to its wintering grounds in southern South America, returning northward from February to May.
A fascinating pattern can be seen by concentrating on the Central Valley of California. The occurrence of Swainson’s Hawks here has been changing in recent years, as the species appears to have taken advantage of the large areas of agriculture that have become available to them within the last century. This year-round animation shows that Swainson’s Hawks arrive here in early March, more than a month earlier than those arriving on the Great Plains. Swainson’s Hawks feed largely on grasshoppers in summer, and the more temperate climate of the Pacific Coast may permit them to return earlier here (a few even winter in California now). But it also seems likely that these birds are not traveling as far south in winter as the more easterly population. Within the past 20-30 years, Swainson’s Hawks have started wintering in the northern hemisphere, and they now occur regularly in winter in Panama, coastal west Mexico, and southern Baja California–all areas that were forest or desert historically, but have recently been cleared and irrigated for agriculture. With hundreds of birds now wintering in the southern Baja California Peninsula, and an increasingly large late February and March migratory passage in southeastern California, it seems likely that these Pacific Swainson’s Hawks are using these new habitats in both summer and winter, and have rapidly evolved a new migratory strategy to take advantage of them.