All,
It has been astutely pointed out (by responders to yesterday's COBIRDS post re the surprising difficulty one can have distinguishing gyrfalcon from prairie falcon in certain circumstances on Trilby Road west of Taft near the Larimer Landfill) that gender is also a factor. This is similar to what we encounter with accipiters, where a small male of one species can approach or even overlap in size and weight a female of a related species. Case in point, I remember many years ago the exceptional birder Brian Gibbons holding an accipiter in his hands at the Lamar RMBO Banding Station and not being able to ID it to species. This, despite measurements, detailed examination of feather coloration, Peter Pyle's book Identification Guide to North American Birds lying on the table next to the bird, etc.
People who know more about gyrs than me think the landfill bird is a male. If the individual prairie that messed with us by sittings on the same pole as the bird we thought was a gyr sat on an hour ago was a female, voila! Instant doubt and confusion. My comment about the difference between the two being "6 inches in length and a gyr being twice the mass of a prairie" applies to extremes apparently not present in the situation Peter and I experienced yesterday. The extremes of difference would occur only if the gyr were female and the prairie was male.
Additionally, IF you see the gyr overhead, which we did not yesterday, it will lack the black axillaries ("armpits") shown by a prairie. In the past, when I have seen the gyr from below, its tail is much longer, flared much more conspicuously, than prairies typically display. If you can view the original photos posted by Andy Bankert back in 2018, this feature is wonderfully evident. Lastly, the wings of the gyr are pointed like the good falcon that it is, but they strike me as much wider at the base than a prairie.
Dave Leatherman
Fort Collins
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