A selection of Kumlien’s Iceland Gulls Larus glaucoides kumlieni, seen in the southern Lake Michigan region, USA, in February 2014. In that region, Iceland Gulls are by default considered to be of the kumlieni type, while Iceland Gull Larus glaucoides glaucoides is treated as a rare visitor.
Adults
In adults, much overlap exists with Thayer’s Gull but look for pale primaries in sitting birds and for the primary pattern in flying individuals.

14 February 2014, Lake County Fairground, Libertyville, Illinois, USA. Adult Kumlien’s (center, left) with American Herring Gulls. Note the faded primaries, slightly lighter mantle, small bill and dark eye.
The following 2 images are of the same individual. The primaries show the most commonest pattern known as Stage 4: “(55%), darker grey or more extensive grey marks on the outer five primaries with complete subterminal bands on zero to three primaries”.
In the case of this individual, P10 and P9 have black on the outer web only, while P8 and P7 have complete subterminal marks while P6 has a very narrow and broken subterminal band.

15 February 2014, North Point Marina, Winthrop Harbor, Illinois, USA. Individual with black on the outer web of P10 and P9, a full band on P8, P7 and P6.

15 February 2014, North Point Marina, Winthrop Harbor, Illinois, USA. The darkness of the mantle is due to poor light conditions and photo editing.
The following 4 images and video are of the same individual.
This one too shows a Stage 4 primary pattern but here P6 also has a distinct subterminal band.
2nd-cycle / 3rd-calendar year
All photos are of the same individual.
1st-cycle / 2nd-calendar year
All photos are of the same individual.
References
Gull Research.org – Adult Kumlien’s Gulls