Birds of North America account notes (compiled by Brainard Palmer-Ball, Jr):

I have attempted to distill a few points from the BNA account for Iceland/Thayer's Gull (combined) that clearly lead me to believe the Ky Dam bird is not safely assignable to one "type" or the other. In fact, it makes me wonder exactly what we DO get here in Kentucky and the rest of the central US. If one is to take a reasonable approach, I think we'd have to say that we are likely getting birds from the most variable portion of the breeding range (i.e. neither clear nominate Iceland Gulls nor clear Thayer's Gulls), but birds that are typically looking more like a Thayer's 'type' with some Iceland 'types' thrown in on occasion, as well as some 'tweeners' that are probably just that. I've always noted that photos of west coast Thayer's make the birds look a lot darker than every one I've ever seen in Ky, so maybe they are ALL 'tweeners' here. It also makes me wonder if we might be missing more of the 'tweeners,' calling them Thayer's types when they could just as likely be Kumlien's types. In this regard, I would have to agree with the theory that the current Ky Dam bird might be more like a "Kumlien's" than a "Thayer's" but who's to know?????

These points are taken from 

Snell, R.R. 2002. Iceland Gull (Larus glaucoides) and Thayer's Gull (Larus thayeri). In the Birds of North America, No 699 (A. Poole and F. Gill, eds.). The Birds of North America, Inc. Philadelphia, PA.

Nominate Iceland Gull (L. g. glaucoides) is defined as those birds breeding along the western side of Greenland south of 70° N and on eastern Greenland. Most but not all nominate Iceland have lighter gray mantles and lack patterning in their wing tips.

Kumlien's Iceland Gull (L. g. kumlieni) is defined as those birds breeding east, south, and southwest of Baffin Island, Digges Sound area, and west to eastern Southhampton Island.  Adult Kumlien's have highly variable wing-tip patterns, sometimes patternless and sometimes like Glaucous-winged Gulls (L. glaucescens). Mantles of these birds are usually darker than those of nominate Icelands.

Thayer's Gull (L. thayeri) is defined as those birds breeding in northwest Greenland, and in the Canadian Arctic north and west of Kumlien's.  Most adult Thayer's in the westernmost portion of their range have more extensively patterned wing-tips and darker mantles than Kumlien's.

The important point here is that both subspecies of Iceland Gulls and Thayer's Gull are technically recognized by WHERE THEY BREED more than the particular degree of melanism in their plumage. This is especially important for the middle-of-the-road birds that we seem to get in the midwest.

Variable plumage and overlap in all morphological characters complicate field identification of many individuals in the Iceland/Thayer's complex. Snell's studies suggest a "continuous phenotypic variation from darkest to lightest extremes in plumage of adults across the breeding range."  Moreover, in the narrow area where the breeding ranges of the two overlap, "there is no evidence of assortative mating: gulls as dark or darker than the type of thayeri bred with others much lighter than the type of kumlieni, including birds lacking visible wing-tip melanism."

As a side note, Snell notes that eye color is not a reliable character in separating adult Kumlien's from adult Thayer's.

Generally, the darkness of the back in adults is proportional to the amount of dark patterning in the wing-tips, but this is not always the case [which may translate into variable primary color relative to body (back) color in immatures -- this is an inference of mine].

Snell makes the point that juvenile birds of both Thayer's and Kumlien's have never been positively identified (i.e. correlated to their natal origin) [which one must think makes identification of any funny looking bird impossible]. To quote, "[m]any identifications are hypothetical, as specific plumage patterns preceding particular adult forms have not been documented with known-age specimens of known origin." He specifically notes high variability in plumage patterns among individuals observed throughout the Great Lakes region.

Snell also notes that "numerous descriptions in literature of subjective impressions of gestalt-like appearance and behavior (i.e., jizz) are based almost entirely on observation of birds of unknown age, unknown sex, and unknown natal origin."  Moreover, he states that he concurs with Zimmer (from Birding 1990) that "our ideas regarding what a 'classic' Thayer's Gull should look like (structurally) are based primarily on the appearance of a female Thayer's." He goes on to say that some female Herrings are as 'cute' looking as Thayer's and some male Thayer's are as large-billed and flat headed as many Herring Gulls. 

In adult birds, the primaries of the lightest Thayer's Gulls broadly overlap Kumlien's. Mantle melanism in adult birds also broadly overlaps between lighter Thayer's and Kumlien's. [Again, I would infer that this likely translates down to relative lightness of primaries and back color in young birds].

Finally, he states that "[g]iven degree of overlap in all characters, the 3 taxa may at best be considered convenient categories for classifying sight records."

Coverage on juvenal plumage (and corresponding first-winter) is scant in the BNA account, but Snell notes that nominate Icelands are generally whitish with distinctive light primaries, back feathers and wing-coverts more or less marked with drab or light brown. Primaries are often distinctly paler than wing-coverts and back feathers.  Most but not all Kumlien's have dusky apical spots on the primaries.  These spots are a subtle feature that are clearly seen in a few published and web photos. Juvenal plumage is thought to be retained (in greater and greater degrees of wear over time) into January or later.

Scapular bars are another feature that vary greatly in the group on juveniles, with 9 of 10 Kumlien's barred, but only 13 of 20 Thayer's barred in one study. The remainder of Snell's discussion deals conservatively with the group as a whole and the variation from light to dark birds.

I would encourage anyone interested in all of this to purchase a copy of the BNA account for Iceland/Thayer's Gull. It should be available through Buteo Books.
 

The February bird (above right) is also an example of a pale juvenile Thayer's.  However, this bird shows more extensive and less clearly-defined pale edges to the primaries, as well as a pale and somewhat ill-defined tail band.   Although some wear/bleaching is evident in the plumage as would be expected in late February, close study showed that it did not appear to be excessive.  Note that the center of the tertials, although pale, still retain the Thayer's-like pattern with solidly-colored centers. 

Although I recorded this bird as a Thayer's, I labelled the images obtained as possible thayeri/glaucoides since I can't rule out some degree of Iceland influence.  I think the appearance of the primaries and tail band may be indicative of such in a bird still maintaining overall Thayer's features when the plumage is not excessively worn.  It is certainly more Kumlien's-like than late-winter juvenile Thayer's we typically see in Kentucky. - David Roemer