I encountered a large, light falcon on Kootenay River Road in Creston. I studies it at quite a distance through the scope, snapped a couple poor shots, but when I attempted to get closer it flew away to the north.
Prairie Falcon was my initial sense. It was light with dark streaking on a very light breast. A dark eye-line dominated the facial markings. There was a noticeable light patch extending back to the nape, but what appeared to be a medium-dark vertical line through the nape. It had thin a light supercillium and a bit of a brownish cap. The moustachial line, however, was not very strong, as I would expect from a Prairie.
I still assumed Prairie when I watched it fly by, but I did not see the dark axillaries and under-wing coverts I expected... they may have been a little darker medium brown, but did not really stand out from the rest of the underwing. The bird was flying low, so it was not a perfect view of the underwing pattern, but no hint of the dark contrasting parts.
Given the unusual face pattern and plain underwing, I'm considering Gyrfalcon. I have a bit of experience with Gyrfalcons from the coast, and I know the plumage is variable. The facial pattern on most Gyrfalcons I've seen is more uniform than this, but I've found some photos and field guides that show more of an eyeline. It seemed large in flight, so it is plausible. On my view of the perched bird I thought it was more of a Prairie/Peregrine size, but in the fuzzy photos I could see that the head might look proportionally small. But if not for the underwing I would be leaning Prairie.
For those of you with more experience with Prairie Falcons, do they ever lack an obvious moustachial stripe? Is the dark on the underwing pattern ever more subtle?
Apologies for the photo quality.