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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