Swainson's hawks are nimble bug hunters

    Swainson's hawk [Photo courtesy hawksaloft.org]

    Given a choice, many teens will opt for pizza. A burger and fries is an acceptable substitute. But a teen that prefers kale or Brussels sprouts is considered odd. Something similar happens in the hawk world.

    Red-tailed hawks nesting in the Rogue Valley love nothing better than fresh ground squirrel for breakfast. For a little variety, rabbits, mice or snakes are acceptable alternatives. This is what redtails and most other Buteos eat — fresh red meat.

    Buteos are a group of closely related hawks. They are large, spend considerable time quietly decorating utility poles and treetops, and frequently soar. There are five species that regularly occur in Oregon. The red-tailed and red-shouldered hawks breed in the valley. Ferruginous and Swainson’s hawks nest in the high desert and grasslands east of the Cascades. The fifth is the rough-legged hawk that visits in winter.

    The seeming oddball in this group of proud hunters is the Swainson’s hawk. Don’t get me wrong, the Swainson’s hawk is a skilled hunter of ground squirrels in the breeding season. But in the off season they feed on foods that would embarrass other Buteos.

    I just returned from a trip to Southern California. It’s been a wet winter, and the desert flowers at Anza Borrego were spectacular. I caught the tail end of this rare bonanza, but it was impressive nonetheless. The other spring event at Anza Borrego doesn’t get the same press, but it is more dependable. Each year more than 10,000 Swainson’s hawks pause in their northern migration to rest and refuel in the desert.

    It’s the fuel that is a bit surprising. It’s not some feisty squirrel or other red-blooded prey, but caterpillars. The three-inch caterpillars are abundant for a brief period during the spring floral display even in years with normal precipitation, and then they quickly vanish. Swainson’s hawks arrive just in time to harvest the bounty. The image of a large hawk on the ground plucking caterpillars from plants just doesn’t fit into my concept of the natural order. It may not be dignified in the eyes of other hawks (or mine), but it provides abundant and nutritious food on their long northern trek from Argentina to places like Central Oregon and British Columbia.

    Before you think this is a quirk, Swainson’s hawks do something similar prior to their southern journey, as well. Soon after young Swainson’s hawks fledge, ground squirrels in central and eastern Oregon begin their seven-month slumber. The principle food source for the hawks just vanishes. What’s a Swainson’s hawk to do? August may not provide many ground squirrels, but there are plenty of grasshoppers. Just ask any farmer or rancher east of the Cascades.

    To a Swainson’s hawk, it’s all the same. One form of protein is as good as another. They come down out of the skies to stand in the fields. I have watched families of Swainson’s hawks dashing about on foot, first in one direction then another in pursuit of the lively, hopping prey. They’re amazingly good at it, and it likely explains why young Swainson’s hawks have a higher survival rate than other Buteos. Grasshoppers are easier to catch than squirrels and rabbits.

    And what do Swainson’s hawks feed upon when in the Argentine pampas? Grasshoppers!

    — Stewart Janes is a biology professor at Southern Oregon University. He can be reached at janes@sou.edu.

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