This smallish Herring Gull-like bird (photographed 26 Dec 2009 at Erie Reservoir in Boulder County, Colorado) resembles Thayer's Gull (Larus thayeri) very closely but the underside of the primary tips is darker than expected. Upper wing-tip pattern is a perfect match for Vega Gull (L. vegae) according to Olsen and Larsson's Gulls of North America, Europe and Asia (corrected edition, 2004) p. 28. Pinker feet, dark eye, and darker gray mantle match both thayeri and vegae. Vega Gull is supposed to match Herring Gull (L. argentatus) in size (officially, vegae is a subspecies of L. argentatus, as is American Herring Gull, L. a. smithsonianus). Vega Gulls breed along the north coast of Siberia and winter in the western Pacific coast (e.g. Japan). The gray-scale of the adult mantle varies geographically, with some populations pale (like L. thayeri??) and some darker (like L. californicus??); darkest gray birds apparently are found at the western and eastern extremes of the breeding range, including St. Lawrence Island, Alaska. Gape and orbital colors are probably useless in December. Could this be an adult female Vega Gull? If so, it would be a paler individual, presumably from Siberia. To my knowledge, this taxon is not officially recognized on the Colorado bird list, although I know of one putative report. Can a hybrid (e.g. Vega x Thayer's, or Herring x Thayer's), or very dark-wing-tipped Thayer's gull be ruled out?
Click on an image to enlarge. Page 2 shows additional images that were overexposed. Page 3 shows these images after autocorrection by MS Picture Manager photo editing software.
