Originally I suspected a Glaucous X something hybrid based on bill structure and color, and head structure. I suspected a first cycle bird based on plumage and dark iris. After reviewing the photos more, and corresponding with experts, the bird appears to be more likely a second cycle bird. Gray feathers are coming in on the mantle, and the primaries look rounder-tipped rather than pointy as first cycle birds have. The overall uniformity in plumage color suggests GWGU X something as well (if not pure GWGU!), though mixed birds can be rather uniform in their second cycle. Nelson's Gulls generally have more patterning on the greater coverts and primaries.
It seems like the best arguments I've heard are for Herring (really Smithsonian, but let's not go there now) Gull X GWGU. Still, I've received more votes for Nelson's than anything else.
Any more ideas about this gull are welcomed. Email us or comment on this post if you have anything to add. As a side note, check out the single dark feather on this bird's scapulars - very strange.
By Luke Musher