Mike Patterson wrote:
>
> Boy, I hate to be the barer of bad news (and I'm sure you'll argue
> with me), but I think you've got a male Northern Harrier here. And you
> wouldn't be the first to do this.
>
> A summer Gyrfalcon in the Pacific Northwest is a really low percentage
> happening, but a white Gryfalcon, well we don't see them much at all in the
> time of year that they are expected. Most of the gyrs we get are brown or
> gray phase birds; heavily marked beneath. They're more likely to be
> confused with Peregrins or Prairie Falcons.
>
> Male Northern Harriers, on the other hand are absolutely white underneath.
> They will often fly in the manner you have described.
>
> >
> >The bird was falcon-shaped with long and pointed wings, bent back at the
> >wrists. It flew with its tail straight back, not fanned. The bird was
> >uniformly white with very little markings. There were no distinguishing
> >marks anywhere on the underside. I did not see the heavy mustache mark
> >that would accompany any other falcon. The bird was larger than a prairie
> >falcon and seemed heavier. It was about the same size as the osprey we saw
> >within 30 minutes. I can find no other bird in my field guides that this
> >could be except a gyrfalcon.
> >
> >If anyone can suggest an alternative id. that fits better, or can find the
> >bird and confirm the sighting, please do!
> >
> >Wendy McCrady
> >mccrady at primenet.com
> >Bisbee, AZ

Two other options to consider. If I recall correctly, I once heard
that a partial albino Red Tail lives in the San Juans.

Another unlikely, but interesting, possibility is a White-Tailed
(a.k.a. Black-Shouldered) Kite. They are slowly spreading their
range and are already known to be in the SW corner of Washington.
I believe that young birds are known to wander far from their
established areas, so the San Juans are a possibility. In fact,
late last summer, I saw a bird in Redmond, WA (East of Seattle)
that fit your description and was very difficult to match with
any of the typical birds in this area. The best match I could
find in all my guide books was a juvenile, male Kite, although
I've never seen before (or since...) in their known range, so
I'm hesitant to make any bold claims.

Dean dean at amc.com