Iceland Gull adult

Began feeding below the dam with many others, though remained at great distance. This image shows the spread upperwing. There is a lot of variability in the pattern of outer primaries in Iceland (both Kumlien's and Thayer's) and in Herring. Though some Herrings have less black and more intrusion of gray on the outer primaries than others, a pure Herring would not have this degree of gray and white on those feathers. The amount of white on the outer two primaries (P9 and P10) is more than usually seen with "pure" Thayer's, and the lack of a small subterminal black bar on P5 is uncommon in Thayer's compared to Kumlien's. Degree of blackness is at the Thayer's end of the spectrum.

To my eye this bird is likely a Thayer's with mild intergradation with Kumlien's.