When I originally spotted the first adult, thoughts of Thayer's popped in my head but were quickly ruled out when noticing other points (not enough black in the primaries, wrong head and bill shape, and eventually getting to see the light eye). I admit that the Thayer's/Kumlien's complex still confuses the crap out of me.
The Icelands were among about 250 Lesser Black-backed Gulls (counts of 257 and 233 were taken at two different times), about 250-300 Ring-billed Gulls, and about 20 Herring Gulls. Around 3:30, most of the gulls lifted off thanks to a Red-tailed Hawk and headed east, but the adult Iceland Gull with the gray primaries was still there when I left with about 100 other gulls.
Other notables included the following:
27 Snow Geese (two with yellow bands marked "12CC" and "51CC")
4 Green-winged Teal
3 Northern Pintail
1 American Coot
2 Tree Swallows.
These two adult birds plus the continuing 1st-winter and 2nd-winter birds produced a site record four Iceland Gulls at one time!