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Are you reading primary feathers lengths as a comparitve measurement? Excellent info. Is that documented anywhere? We are still trying to sort out the adult/immature issue. One bird book I have says the blue/gray feet are a immature trait and another says the blue/gray bill is a adult thing. Was told the easy way to tell them from a Peregrine is the yellow coloration around the eye/beak area on a Peregrine. Thanks for the info.
Yes, the info above pertains to the relative length of the 4 outermost primaries...sorry I wasn't clear there. Re documentation: many years ago raptor ID was my day job and I'm most comfortable when I can draw upon first hand experience, but some things, as here, are out of my experience. Also I have forgotten a lot , and the database is constantly growing and changing, so I lean ever more heavily on the literature available. In this case I went first to an old standby: Handbook of North American Birds. Volumes 4 & 5 cover raptors and the info above is cited on p. 1 of the Gyrfalcon chapter (vol 5). But like me this is old material, so I also like to check in with the Cornell Labratory of Orthnithology, which I use as my primary reference for current data. The formula under discussion is also cited there. (Cornell/AOU is a subscription service...about $35 or $40 for one year, IIRC). I access it at https://bna.birds.cornell.edu/bna.
As for bill, cere, & foot coloration: Both falcons show considerable variation by geography, overall color group, and age. The literature is not providing me an easy way to distinguish all individuals using only these characteristics, and I lack the field experience to make an informed statement pro or con. That's why I offered the feather length formula: reading it in flight is more challenging than having a bird in hand, but there is no mention of variabilty across species.