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
>
>

--
********************************* One of us might lose his hair
* Mike Patterson, Astoria, OR * but you're reminded that it once was ther
* mpatters at orednet.org * from the embryonic whale
http://www.pacifier.com/~mpatters to the monkey with no tail -- TMBG