Wouldn't it be wonderful if there actually was a "clincher" field mark
for Slaty-backed Gulls? Charlie quite rightly points out that the p10
mirror can be quite large in adult Western Gulls as the photos in the
URL he posted demonstrate.

But the p10 is rarely that large in 3rd cycle or younger Western Gulls.
I have taken the liberty of combining Matt's enlarged photo primaries
only photo and a primaries only photo of the pale-eyed Western I posted
in my original "beware" post. The differences demonstrated in these
two photos are far closer to the norm in 3rd cycle gulls.

http://home.pacifier.com/~neawanna/temp/wegu_sbgu_p10.JPG

Mind you, you still need the pale eyes and the bright pink feet, and the
darker than the surround Western Gulls mantle and the fine streaking
to the head and all that other stuff. But when you're looking at a
photograph, especially a less than perfect one, this is a far less
subjective field mark than most of those other things.


>Mr Wright opined:
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.

--
Mike Patterson
Astoria, OR
celata at pacifier.com

When bad photos happen to good birds
http://www.surfbirds.com/blogs/mbalame/archives/2008/03/photo20080313.html