Tweets,
My observations of the Slaty-backed in Tacoma were this. The bird had a
very dark mantle, this alone would not identify this bird. There were a
couple of westerns that appeared almost as nearly dark mantled as the
Slaty-backed. The streaking on the head was very fine. When the sun shown
on the beak it had a yellowish-white bill very noticeable compared to the
other gulls. At close range the pale eye was distinct though I'm not sure
its a valid ID mark after reading some of the literature. THe really
noticable trait about the head to me was the resemblance of the thayer's
head. For the size of bird it was it had a smallish head (somewhat
rounded) and a slender beak, I hesitate to call it skinny because when
compared to the westerns it was not as bulky, but still a fair sized beak.
In flight I was able to view the *string of pearls* on the wing, which was
enhanced by the dark leading strip in the primaries in the underwing.
These are my observations of the bird and my analysis using literature
sources such as Grant, Harrison ect.. Since this was my first Slaty-backed
the relevance of each trait is subjective and which identification trait
holds more importance should be left up to people with far more expertise
than me, which there are many.
Thanks,

Scott Downes
sdownes at u.washington.edu
Seattle WA