Swainson's Hawks Are Back
Swainson’s Hawks are back in California. I just encountered a mating pair in the city of Brentwood right next to the field where a few pairs of Burrowing Owls are nesting (see previous post on 4/7/15). It’s a treat to see these majestic raptors come back every year considering they just flew roughly 10,000 Km all the way from South America (most likely Argentina).
Not surprisingly, I found them on a large vacant lot where hay has just been cleared, and therefore, blown the cover for a colony of at least 2,000 ground squirrels. Swainson’s have adapted to live around people (here and in Argentina) and often follow farm and field disturbance activities to make the most of their hunting efforts. In this case, an unlimited supply of ground squirrels with no vegetation to hide from aerial surveillance and attacks. Along with their clever hunting tactics, one of the most striking aspects of Swainson’s Hawk ecology is their choice of prey at their breeding vs non-breeding grounds.
Swainson’s Hawks often hunt on he ground. This pair was hunting ground squirrels by standing right next to their burrows waiting for the right time to strike.
To raise their young in North America Swainson’s Hawks focus on hunting mammals and other vertebrates such as reptiles and birds. However, during their non-breeding season and though most of the austral summer in Argentina, they feed almost exclusively on insects, particularly grasshoppers. Amazingly, the trend of switching from vertebrates when breeding to primarily insects at wintering grounds is common practice for the world’s 6 complete raptor migrants (Swainson’s Hawk, Mississippi Kite, Red-footed Falcon, Amur Falcon, Sooty Falcon and Eleonora’s Falcon) that breed entirely north of the equator and overwinter almost entirely in the Southern Hemisphere.
Looking at this mating pair of Swainson’s Hawks hunting young ground squirrels on this field really shows why they’re here in the Central Valley. It is all about ensuring an abundant food supply during their relatively short breeding season. But why travel so far to South America once they’re done raising young? Perhaps, without the high food demands and stress of breeding, a warm summer in Argentina with an endless supply of easy-to-catch grasshoppers is just the perfect vacation spot and worth the long flight.
Photo (right): Complete migrants such as Swainson’s Hawks have a relatively short breeding season (as compared with other migratory raptors). They get right down to business when they arrive to their breeding grounds.
If you’re interested in seeing these raptors, they have been hunting regularly on a vacant lot right across Pioneer Elementary School in Brentwood, CA (Shady Willow Ln. between Amber Ln. and Grant St.). If you go, please stay on the sidewalk (bring a spotting scope) and do not interfere with their hunting efforts by keeping away (at least 50 yd.). Remember, they have a short breeding season and need to make the most of if before heading south.