Hi Dan and Tweeters,
I've banded 8 gyrs here in western WA (Skagit and Whatcom counties) since
1973. All of them were females, 3 adults and 5 immatures. Two of these were
caught in later years showing that they are philopatric. At least some that
establish a wintering area as immatures will return there as adults, a
behavior typical for many falcons. That Snohomish gyr probably has been or
will be in the same area (using the same perches) for years.
I would say that it is going to be very difficult to determine the sex of
gyrs in the field. I know it is for me. The reason for the difficulty is
that there is overlap in both their wing and tail measurements, somewhat
like Red-tails and Rough-legs. The one exception to me is if you happen to
see one of those really huge females which occur here every so often. They
are pretty obviously females but you need alot of comparative experience to
call it. For example, falconers who fly gyrs on a regular basis are usually
able to tell them apart readily.
The general thought is that there is a bias towards females wintering in
the lower 48 states, probably due to the adult males remaining at or near
their eyries (nests) in the north. I have seen one or two birds over the
years that I thought might be males, based on their small size, but I could
not be positive.
Nancy Clum and Tom Cade have done a great job on gyrs in the Birds of
North America (BNA) series. There is alot of good information there.

Bud Anderson
Falcon Research Group
Box 248
Bow, WA. 98232
360.757.1911