This white winged gull was seen on the Niagara River in Niagara Falls, NY around 2pm today. Only have seen a couple of Iceland Gulls in my time and not a Glaucous yet.
See the three pictures entitled "ww_gull1-3".
My first impression was Glaucous as it looked bulky from a distance.
Then I was able to glean some context by the two other nearby gulls shown in two of the pictures.
I created two scenarios in my head: one at the time, and one afterwards. In both scenarios I perceive the white winged gull to be middle sized between among the three.
The first scenario at the time of sighting was that the smallest gull was a RBGU, and therefore the black-backed was a LBGU, since it doesn't totally dwarf the RBGU. But that would put the white winged gull at an Iceland size, i.e., bigger than a RBGU and around the same size as a LBBG.
Afterwards, when Merlin photo ID said GLGU, it seemed possible that the black-backed was a GBGU, and the RBGU actually a HEGU, which would put the white winged gull at GLGU size, i.e., bigger than a HEGU, smaller than a GBGU.
Anyway, you can see I'm a bit stuck and feel silly in retrospect being unsure of RBGU vs. HEGU.