This gull is a bit more confusing because it is much more darkly marked than the other two, and my first impression of this bird was a pale Thayer's Gull (Larus thayeri), a regular winter visitor in Colorado. However, the paling yet mostly dark bill indicates a first cycle bird now in advanced pre-alternate molt (normally not advanced in thayeri in mid-April). Closer inspection shows barring on tail, traces of barring on the faded tertials, and broad white tips to the primaries, all features favoring kumlieni. Indeed this bird is reminiscent of the dark kumlieni that I reported from Larimer County on 14 Dec 2006, but now more advanced in molt and more faded (see photos at https://pbase.com/quetzal/palegull12142006 ).

The first photo in this series shows the gull in company of many California Gulls and one Herring Gull, and is useful for seeing the intermediate size and bulk of this bird compared with those species.