Slaty-backed hybrid???

[Mystery bird (right) with 2nd-cycle Glaucous-winged Gull]

Last August I visited Sandspit (Haida Gwaii) for a couple days and on one morning I noticed this interesting gull. I initially thought it was a Slaty-backed Gull since from a distance it looked a lot darker than nearby Glaucous-winged Gulls and had obvious dark vertical streaking on the hind-neck and dark shadowing around the eye (a Slaty-back character in basic plumage). As I got closer however (I didn't have a scope so I had to sneak across Shingle Bay to get to it), I noticed that the mantle was not as dark as I had first thought-- perhaps as dark as occidentalis Western Gull, but not the obvious "slaty" colour of classic SBGUs. It also seemed on the bulky-side for SB--although slimmer than GWs when facing straight-on.

Check out these photos and let me know what you think. Although this bird might appear to resemble third-cycle Western x GW, the heavy vertical streaking (not well-portrayed in the photos), parallel-edge to the bill, and head-shape, seem to suggest otherwise?? I don't have any decent shots of the bird in flight but I can tell you that it did not have the so-called "String-of-pearls" typical of adult SBGU-- since this is a 3rd-cycle bird however, I'm not sure how reliable that mark would be; also the inner webs of the outer primaries appeared light bluish-brown (WEGU is black). The eyes of this bird were amber/light brown-- SBGU is usually but not always bright yellow. GWGU x SBGU seems like a possibility to me but I have zero experience with SBGU so would appreciate more input...


[Above: Mystery bird with presumed hyb. 3rd-cycle Herring x Glaucous-winged Gull]

[Above: Mystery bird with presumed adult Hyb. Herring x Glaucous-winged Gull]

[Above: Mystery bird with adult Glaucous-winged Gull]

When I visited Sandspit in August I didn't see a single 'obvious' Western x Glaucous-wing hybrid; instead pure Glaucous-wings dominated followed by presumed Glaucous-wing x Herring hybrids. When I returned in November however, Western hybrids were quite common.

Thoughts?