Swainson's Hawk
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.
