This morning I watched an adult slaty-backed gull in the parking lot at the rendering plant in Tacoma, from 10:20 a.m. until 11 a.m., at which point it flew off.

There were several adult western gulls that in this overcast lighting looked dark mantled, but the slaty-backed was much darker, had a very pale eye (like a herring--not like the pale-eyed westerns, which really have more of an amber iris with a dark pupil), and it had a much slimmer and slighter bill than a western gull has.
To get to the rendering plant, take exit #135 off I-5, which is the Puyallup and Portland Avenue exit. Head north on Portland Avenue, turn left on Lincoln Avenue, and the rendering plant is on the right, east and across the railroad tracks from the parking area for the Gog-Li-Hi-Te wetland.

The gull could be anywhere in this area: at the rendering plant, on one of the rooftops, or on the pond at the wetland. These are all areas where a slaty-backed has been observed in previous winters.

Mike Donahue
Seattle