I stopped at a few lakefront spots on Sunday morning to see what I could find. Diversity was low, but it was obvious the gulls were restless. I found 1 adult Kumlien's and 6 Thayer's Gulls (5 adults and a 3rd/4th cycle type). Quite frankly, I was under-dressed and uder-prepared for the cold blast, as I was working my way up north for a meeting. Much more time was devoted to scanning from the warmth of my car, rather than photography.
My only sub-adult Thayer's of the day - all black base to the bill with a yellow tip. The iris is a light honey color and the wingtip is with plenty of pigment.
And to end with, an adult Thayer's with a relatively wide trailing edge. I don't think I've seen a THGU with inner primaries that have such thick white tips. Notice how the white cuts into the shaft of p2-p5 (very Slaty-backed and Vega like):