Little Gull at Windsor Lake, Weld County, Colorado, 2 Dec 2012
Sorry for the horrible photos, but it was the only documentation I could get. This is a very rare bird in Colorado. The bold black M mark on the wings indicates a first winter bird. The gray nape, fairly extensive black cap, and dark smudge on shoulder indicate that it is still molting from juvenal plumage.
Click on thumbnails to enlarge and for commentary.