This gull, photographed 27 March 2010 in Weld County, Colorado (USA), shows features of adult Vega Gull including dark eye like Thayer's Gull but blackish undersides of the outer primaries (wingtips) as in Herring Gull. The blackish smudge anterior to the gonys indicates that the bird is not quite in breeding condition, and therefore possibly a 4th-cycle subadult. Vega records in Colorado are all currently considered hypothetical. The shade of gray of the mantle (on the pale side for Vega) would indicate "birulai" subspecies that breeds in northeast Siberia. For other candidates of "birulai" Vega from Colorado earlier this winter, see https://pbase.com/quetzal/mysterygullq and https://pbase.com/quetzal/mysterygullo.
