Umiddelbart ser det ikke ud som om at den har helt mørk hale eller mørke spidser på undersiden af håndsvingfjerene som danner et mørkt bånd, hvilket er typisk for Thayers, men dog ikke nødvendigt, så det bliver nok svært.
Mvh.

Hi Silas!

I would agree with Silas on his ending lines! When you try to get involved with some theyeri/kumlieni you usually end up realising that you are treading on thin ice!
Silas pic alone doesen´t offer specific clues to whether this gull is a thayeri or kumlieni, glaucoides would be out of the equation due to pattern described but does it exclude a dark kumlieni? In the upper right pic the bill looks relatively heavy and two-toned with quite an obvious whitish tip and if that tip was for real, it´s not what I would assosiate with a first cycle bird within these sp/ssp. The undertail coverts - just visible on the upper right pic loks rather sparsely barred for a thayeri, even for some kumlieni but this is a rather variable character I guess.
Further more, does the precense or absence of an obvious, or less so, dark/darkish trailing edge to the outer primaries on the underwing account for a positive id in any specific direction?

Some kumlieni types

Some glaucoides & kumlieni types

Further sets

Some more

Peter's

JanJ

Dette er en ret mørk kumlieni. Disse glider næsten græseløst over i Thayer´s Måger, der dog kan bestemmes på en kombination af karakterer, som generelt mørkere helheldsindtryk, mørk terneagtig bagkant på hånden og mørk hale (det sidste giver mindelser til smithsonianus, omend grundfarven er lysere bruntonet).
Fuglen her mangler mørk bagkant på hånden, svingfjerene er for lyse og halen har for høj andel af lyst. Men en kulmlieni, si!

Thanks for the comments and the courage to tread on thin ice!

In deed the dark trailing egde on the outer primaries appears obvious on thayer's gull, and this gull lacks this egde - if that is a diagnostic caracter this gull must be a dark kumliens gull as Klaus points out. This would be the 6th kumliens gull in a two weeks here - but this is the darkest I've ever seen.

Silas