It was first identified as a Thayer's Gull, and I initially had no doubt about that. But after a few minutes of looking at this bird, I began scratching my head. I soon began calling it the "Herring-Thayer's" in the field and I became uncertain of its identity the more I observed it. Although the wingtip pattern is thayeri-like, the subterminal band on P5 is w-shaped and screams Herring Gull to me. Thayer's can show this sometimes, but the pattern is standard on American Herrings.
Other troubling features include the extent of black on the subterminal portion of the bill, coupled with the relatively pale eye, blocky head, and relatively stout bill. Although all of these features can be found on Thayer's, it troubles me to see them combined on a single Thayer's.
Body size was medium-like and a tad smaller than what is expected for male Herrings, but a tad larger than female Herrings - in short, body size was perfect for Thayer's. Another mark in favor of Thayer's is the purplish-pink orbital ring. An odd mix of marks at best.
This bird may very well be a Thayer's Gull, but the overall impression I have of the face is contrary of what I understand to be a Thayer's jizz. I've never attempted to identify a gull as a Thayer's x Herring hybrid, and if I was going to, this would be the bird to start with! As Todd Easterla put it, "There must be some dark corner on this Earth where the two have hybridized".