Saturday, March 14, 2015

The arrival of Swainson's Hawks

As the time approaches in which the first Swainson's Hawk of spring is eagerly anticipated, I feel that a caution is warranted.  While the identification of Swainson's Hawk is generally considered to be fairly straightforward, there are a few situations in which identifying a bird as a Swainson's Hawk is less than straightforward.  In all of these situations, the difficulty lies in not understanding the whole range of plumage possibilities in other species.  I have treated one of these:  light juvenile Swainson's Hawk vs. light adult male Rough-legged Hawk.  Another is the Bald Eagle plumage termed "White Belly I" by Clark and Wheeler, in which individuals have white wing linings and belly contrasting with dark flight feathers.  The third situation, and the impetus for this short essay, involves certain adult Red-tailed Hawks.

Steve Mlodinow sent me a photograph this morning (Fig. 1) that seems to show a buteo with a reddish bib, nearly unmarked underparts, and very little in the way of white markings on the scapulars.  Looking at the picture, I can certainly see how someone could go astray in the ID process!

Figure 1. Adult Red-tailed Hawk, Union Reservoir, Weld Co., CO, 9 March 2015. Photo by Steven G. Mlodinow.

However, with a bit closer scrutiny, we can see that it is just the sides of the breast that are reddish; the center is white.  Many adult Eastern (subspecies borealis) Red-tailed Hawks, at least those breeding at the latitude of Colorado (very few at the latitude of Michigan; pers. obs.), have little in the way of a belly band, thus enhancing the similarity with Swainson's Hawk.

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