Return of the Swainson’s Hawk

On the 8th of March, 2012, I saw my first Swainson’s Hawk of the year!  I always look forward to when these birds return to central California.  This bird was calling from a perch at the top of a redwood tree on campus.  It is a spot that seems to be favored by Swainson’s Hawks, as last year a pair set up a territory and nested in the same group of trees.  I wonder if this is one of the same birds.  Now, about a week after that first bird arrived, Swainson’s Hawks fill the skies over Davis as the bulk of the returning population begin to arrive.

Swainson’s Hawks undertake one of the longest migratory routes of any raptor in North America.  They leave from the plains of the continental USA and southern Canada in the early northern hemisphere fall and fly to Argentina.  There, they spend the southern hemisphere summer eating mostly insects.  Interestingly, not all the Swainson’s Hawks go on this long migration.  The birds that breed in California, which is a population that is somewhat separate from the rest of the species, have been found to generally only go as far as Baja California, Mexico.  What makes this group stop so much sooner than the birds that breed in the great plains?  Do any birds from the California population go to Argentina?  Do any birds from the great plains stop in Baja?  I don’t think anyone knows the answers to these questions, but they would be awfully cool to find out!