Pacific Northwest Gulls & Terns

Slaty-backed Gull ?

Slaty-backed Gull at the pellet plant, Sauvie Island.........February, 1998.

Is this an example of the Slaty-backed Gull seen at the pellet plant on Sauvie Island? It was definitely darker on the back than all the other gulls that were there. The "experts" who were busy watching it seemed to think that it was, but I guess I'm not convinced. It looks a lot like a dark Western Gull to me. What are your thoughts?
Paul Slichter