Hi all,
Hopefully people don't think this is beating a dead horse (hey, it's gulls),
but I must disagree with Mike Patterson that a large mirror on P10 is the
clincher on this bird. Western and Glaucous-winged both have a quite large
mirror on P10, like Slaty-backed. Take a quick look at
http://www.garrulax.com/adultwegu to see roughly how large the mirror can
look from below (especially the last photo). It is true that adult
Slaty-backed Gull has a far more complex primary pattern, which appears
something like Thayer's Gull with white crescents on the folded wing.
However, on younger birds the white can be much more limited, and the P10
mirror is highly variable in 3rd cycle Western Gulls. The photos (as
excellent and sufficient as they are in other respects!) don't really show
enough detail to analyze the primary pattern here, aside from there being a
P10 mirror.

I also really want to thank Bill Shelmerdine for taking his time and writing
an exquisite explanation of just what makes this 3rd cycle bird
(photographed by Matt Pike) such a fine example of a Slaty-backed Gull!

I also want to point out that a lot of our recent understanding of 2nd and
3rd winter Slaty-backed Gull identification is thanks to birds photographed
and meticulously identified at Half Moon Bay in California by Alvaro
Jaramillo and numerous others. Looking through his analyses
(http://www.coastside.net/chucao/gulls/Slatyback.htm) has been very helpful
in learning what to look for. It's actually a bit surprising how high our
proportion of adults to younger birds is here in Washington. In part, this
is a biased record because adults are more distinctive, and it would be a
great idea to keep looking for potential younger birds.

Personally, I can't wait to get back out there!

Cheers
Charlie Wright
Bonney Lake, Washington