Dave, thank you for your excellent comparison of markings, a fine discussion and the two helpful photographs. If I may further add to your description, as a falconer and someone who has worked with males and females of both species:
I've been told by several experienced raptor watchers who've seen the Ft. Collins gyrfalcon in person that it is a male. If the prairie falcon appears to be about the same size when looking through binoculars, or even with the naked eye, then it is likely a female. The healthy flying weight of a female prairie falcon is in the range of 27-30 oz. The equivalent for a male gyrfalcon ranges from about 32-37 oz. Two falcons so close in weight would almost certainly appear to be the same size when viewed perched on a pole, etc. My point being, this can definitely be a tough call if lighting and other conditions aren't ideal…and both falcons are very close in size.
Good birding to all,
~Anne Price
Littleton, JeffCo
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