My best Thayer's gull memory — I'll bet you don't have one of those — is from a day in Grand Marais, down by the harbor. Two knowledgeable birders were debating about a gull sitting with hundreds of other gulls on the breakwater.
Was the gull in question a Thayer's gull or perhaps a herring gull? Or an Iceland gull? The telling points of identification are subtle.
Slightly smaller bill, slightly smaller body, slight plumage differences, and a dark iris. Herring or Iceland gulls, for instance, have light irises. You wanted to look this gull right in the eye.
I don't remember the question being resolved. I probably wandered away.
The question, asked by many birders over the years, might become moot. Thayer's gull could become kaput. Maybe.
No, not an environmental catastrophe. It is a proposal to be voted on by the American Ornithological Society's North and Middle American Classification Committee. There is such a thing, and in the biological world it is important.
Because a Thayer's gull very closely resembles both herring and Iceland gulls, it could be lumped, becoming one with the Iceland gull.
There have been no genetic studies. That probably would be definitive. I would guess that no Ph.D. candidate has ever chosen this question as a thesis topic. We are left with arguing.