Eye color
The taxon is pale in all plumage, with a remarkably variable amount of pigment in the primaries. Individuals range from completely white-winged (indistinguishable from nominate "L. glaucoides" Iceland gull) to so dark in the wings as to be indistinguishable from Thayer's gull. Eye color is also variable, from pale yellow to dark brown. Such remarkable variation seems to lend credence to the belief that Kumlien's gull is in fact a hybrid swarm.