This large second-cycle gull was at Warren Lake in Fort Collins, Colorado on 14 November 2009. It was the largest gull in the flock, which included Ring-billed, California and Herring Gulls. This gull seems to have characteristics of Glaucous Gull (bill) but also many features of a dark-backed gull (fairly uniform dark brown upperparts). The all dark tail and pail webs of the primaries visible on the underside of the wing suggest Slaty-backed Gull as the candidate. Thus one possible identity of this bird is a hybrid Glaucous x Slaty-backed Gull, or perhaps a better way to phrase it would be a Slaty-backed Gull with some Glaucous Gull genes, as the bill is the only Glaucous feature.
Another possibility (probably more likely) is a Herring x Great Black-backed Gull, or even Nelson's x GBBG. Nelson's is a Herring x Glaucous hybrid, but Nelson's could explain the lone Glaucous character. Presumably Nelson's is a fertile hybrid (given the close relatedness of gull species). What about Nelson's x Slaty-backed Gull? This might make more sense as they might be breeding in the same area of Alaska or Siberia.
