Thursday, April 26, 2007

Return of the Swainson's Hawks

This morning I saw the first Swainson's Hawks (Buteo swainsoni) of the year. These beautiful hawks breed in western North America, and then almost the entire population migrates to southern South America (mainly Argentina). What a peregrination! For the birds coming from Canada, this 12,000-mile (round-trip) excursion represents the longest migration of any raptor except the Arctic Peregrine.

This species has also experienced significant population declines, and its numbers may now be only about 10% of historical numbers. Reasons for this include habitat loss, direct persecution (especially shooting), and pesticides. A critical instance of the latter was discovered about ten years ago, when researchers (Brian Woodbridge and Marc Bechard) were able to track Swainson's Hawks (through the use of satellite radiotelemetry) to their exact "wintering" locales in Argentina. What they found was massive die-offs (6000 in 1995 and 1996), with these hawks succumbing within minutes of being sprayed with extremely lethal pesticides as they foraged on grasshoppers in alfalfa and sunflower fields.

Where we live is on the western edge of the Oregon range of Swainson's Hawks. Here, nests are found near alfalfa fields, and generally where there is a mix of native vegetation and agricultural lands. As relatively late spring arrivals, these hawks will be a good month behind resident species (like Red-tailed Hawks) in their breeding. They'll lay a single clutch of 1-4 eggs, and, if all goes well, two or three young will fledge in July. Throughout the breeding season, the adults will be eating and feeding to their young the Pocket Gophers, and Merriam's and Townsend's Ground Squirrels that feast on those alfalfa fields, the hawks thus allying themselves with the local farmers in controlling these species. In late summer, they will begin to come together in large flocks, and will switch to the largely insectivorous diet that they maintain throughout the non-breeding season.

And guess what? I'll be watching a nest or two in my area, hoping to learn more about these long-distance migrants and how they're faring.

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