Upperparts are 1-1.5 shades darker than Herring, and certainly closer in color to that species than Great Black-backed Gull. It had a piggish look to the face with a large blocky head, giving the impression of GBBG. The bill is strong and thick all throughout its entire length like many GBBG. Body size was closer to Herring, but it appeared long-legged when walking with a front-heavy posture, holding its back horizontal.
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| Lake County, Illinois. 03 January 2018. |
I briefly observed the open wing which only displayed a mirror on p10 with a moderate amount of black on the inner webs of p8-p9. The wingtip shows more black than most other putative Great Lakes Gulls I've seen. Those individuals, incidentally, were also darker with upperparts more intermediate or tending toward GBBG. Thus, I'm assuming the wingtip is due to more Herring influence.
The Lake Michigan region seems to be reporting about 10 Great Lakes Gulls per winter season. I have only seen this hybrid once in the breeding season, an adult in Manitowoc, Wisconsin (July).
With reports of this hybrid - and many other putative hybrids - being predominately of adult individuals, I do wonder what 1st cycle Great Lakes Gulls look like.