Observed 4 Jan 2014 at Aurora Reservoir, Colorado, late in the afternoon under overcast skies. It spent most of the time sleeping, but the brief times it lifted its head, the bill was noticeably heavier than those of American Herring Gulls in the flock (not shown). First seen head on (not shown), and seemed quite pale and splotchy on the breast and head area. Later, from the side (shown below), the wing panel was very pale and washed mostly whitish, but pattern was not crisp marbling as in 1st-cycle Thayer's Gull. The scapulars were mostly 2nd generation (or 3rd generation?), with some gray that appeared darker than expected for American Herring Gull. The tertials and primaries were dark, but not black like typical Herring. When it flew off to roost, I focused on its wings and noticed a dark secondary bar, obvious pale window in the inner primaries, and dark outer primaries, suggesting the wing pattern of American Herring Gull. I thought the pale inner primary window was more obvious than typical 1st-cycle Herring. The photos added on the second row show a Thayer's-like wing pattern in flight.

What is this bird? Perhaps a pale American Herring (the default choice), or perhaps a hybrid such as Glaucous-winged x Herring. Another possibility is pale first-cycle Slaty-backed Gull. Very similar birds of this species are shown in Figures 30.8 and 30.9 of Howell and Dunn's Gulls of the Americas (p. 214). The spread wing pattern is typical of Slaty-backed, with pale inner webs of outer primaries. Note the beginning of a dark secondary skirt extending from the tertials, visible on the sitting bird in the first and last image, indicating long secondaries typical of Western, Glaucous-winged and Slaty-backed. Signed comments are always welcome.

Images are digiscoped and lightened. First row are mine. Second row (not digiscoped, added 6 Jan) are from David Dowell. These were edited to remove blue hue. Click on an image to enlarge.