Fig 1) Best described as a giant blurry Thayer's Gull, note the rounded head, the bill shape and color, the folded primaries, and the messy patterning on the upperparts (including the greater coverts and tertial tips).

Fig 2) Note the Thayer's like pattern on the flight feathers, with dark outer webs and pale inner webs all the way out to P10. Slaty-backed has more solid (and usually darker) P9-10 and more pronounced terminal spots on the inner primaries.

Fig 3) The darkest colors on this bird, on the outer webs of the flight feathers and on the tail, are a fairly washed brown. Slaty-backed is probably consistently darker than this bird.