Summer’s End — Swainson’s Hawks

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One of the sure signs that summer’s end is near is the beginning of the fall bird migration.  It may still be 100 degrees here, but the migrating birds foretell cooler weather.

The other day I went into the grassy plains southeast of Tucson with a friend, looking for migrating Swainson’s Hawks.  These large raptors are champion migrators: in late summer every year flocks of thousands of these hawks pass through the Tucson area from their summer homes on the northern plains in the western United States and Canada to their winter range in Argentina.  They stop over in Arizona to fatten up on insects before resuming their journeys.  It’s not unusual to see dozens or even hundreds of Swainson’s Hawks orbiting a field as they feast on grasshoppers.

We saw only a few birds.  Some Swainson’s Hawks spend their summers in Arizona so it wasn’t possible to say whether the ones that we saw were the vanguard of the migration or local summer residents.  We’ll almost certainly start seeing the big flocks in a week or two.

Swainson’s Hawks superficially resemble their cousins, Red-tailed Hawks.  They’re about the same size, with Swainson’s Hawks being just marginally smaller.  From a distance, when perched, the two species can look deceptively similar.  Up close, however, the distinctive characteristics of Swainson’s Hawks become much more apparent.

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The pale gray-brown bib on this bird is very typical of a Swainson’s Hawk.  So is the bright yellow “cere” behind the bird’s beak.  Individual Swainson’s Hawks display a lot of color variation but that bib (even if the color varies from bird to bird) and the cere are telling characteristics.

I actually find it easier sometimes to distinguish the two species from each other when they are in the air.  Here’s a typical southwestern Red Tail.

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The pale breast and the rusty tail are distinguishing characteristics.  Red Tails have broader and somewhat shorter wings than Swainson’s Hawks.  Also, Red Tails characteristically have darkened areas on the foreparts of the undersides of their wings.  Overall, this bird also looks a bit chunkier to me than a Swainson’s Hawk.

Now, here’s a Swainson’s Hawk.

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There are several differences between this bird and a Red-tailed Hawk that are apparent from this image.  First, the hawk doesn’t have a red tail.  Its wings are longer and narrower in proportion to its body than a Red Tail’s wings.  There are no dark patches on the foreparts of the Swainson’s Hawk’s wings but, unlike the Red Tail’s wings, the rear area of the Swainson’s Hawk’s wings is dark.  This bird sports a typical Swainson’s Hawk’s bib.  And, finally, and although it’s somewhat subjective, this bird strikes me as being a bit less robust than a Red Tail.

Now, candidly, these differences are not all that obvious on first impression and, as I’ve said, these species are undoubtedly related.  It is amazing, however, how different these birds’ lifestyles are.  Many Red Tails migrate but few, if any, are the champion migrators that are Swainson’s Hawks.  And, Red Tails do not congregate in gigantic flocks during their migration.

Images made with a Canon 5Diii, 400 DO.  The first and third images were made at aperture priority setting, ISO 500, f6.3, 1/1600 (first image), 1/1250 (third image).  The second image was made at M setting, ISO 320, f7.1 @ 1/1250.

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