This adult possible Kumlien's Gull (Larus glaucoides kumlieni) was photographed on the Texas Coastal Plain near Galveston, Texas in late February 2012. The bill seems rather short/small, the head rounded, the eye perhaps pale, and the mantle tone a bit paler than nearby RBGUs. The mirror on P9 seems to extend to the outer web, and there is dark on only the outer five primaries:
In the crop below I took a small square of upperwing gray from the bird and copied it into the same position on the wing of three nearby Ring-billed Gulls; it seems to be a bit paler than all of them:
