This large pale-winged gull was at Horsetooth Reservoir in Larimer County (near Fort Collins), Colorado on 18 November 2008. Other than myself, it was also seen by Arvind Panjabi, David Steingraeber, Andrew Spencer and Cole Wild. Andrew provided these excellent digiscoped images. The gull appears to be in its 3rd year. Possible identities that have been discussed include Glaucous-winged Gull, Thayer's Gull, Kumlien's Iceland Gull, Herring Gull, or a hybrid combination of the above (;-). Anyone is welcome and encouraged to comment on its identity on this website (please sign your comment).
My first impression seeing this bird in the distance was adult Glaucous-winged Gull (GWGU, Larus glaucescens), a vagrant in Colorado. Focusing on age first, closer inspection revealed subadult features such as pale yellow bill (not bright yellow), blackish gonydeal mark, very slight brownish wash on wing coverts and tertials and rectrices, and very narrow pale tips to pale grayish-brown primaries (rather than mantle-gray primaries tipped with crisp white spots. After consulting texts (particularly Olsen and Larsson's Gulls of North America, Europe and Asia, 2003), I believe this is a 4th cycle bird, and presumably being from the far north, a somewhat retarded 4th cycle compared to American Herring or Lesser Black-backed Gulls. Is this a straightforward 4th cycle GWGU? Its size is on the very small side but appeared larger than a juvenile Thayer's Gull beside it. According to other observers (Spencer and Wild), it was intermediate in size between two Herring Gulls in its company at one point. The head seems small and round for a typical GWGU but within limits for a small female GWGU, and a bit too blocky and large for typical Thayer's and Kumlien's Iceland Gulls. Similarly the bill runs small for typical GWGU and large for typical Thayer's/Kumlien's Gulls. Same is true for the gonydeal angle of the bill. Eye color is a match for GWGU but on the pale side for typical Thayer's and on the dark side for a typical 4th cycle Kumliens Iceland. The expected pattern on the primaries is not descrbed for a retarded 4th cycle bird, but the pattern on this bird seems reasonable for such a bird (GWGU); it is also a good match for a photo of 3rd cycle primaries of Kumlien's Iceland Gull. The wings appear long for typical GWGU but the extension of 2 primary tips beyond the tail on this bird matches some photos. Could a hybrid combination match these characters? Probably yes (and therefore field identification of this bird beyond any doubts is impossible), but with all field marks within range of a pure-bred GWGU, I favor naming it such. I would feel differently about this if one of its traits was firmly outside limits for GWGU, such as a bright yellow eye, or blackish primaries, etc., but that does not seem to be the case with this bird.
Nick Komar
Fort Collins CO