This gallery tries to photodocument at least 10 individual Thayer's Gulls (THGU, Larus thayeri) present among the gull flock at Horsetooth Reservoir in Larimer County (northern Colorado, USA) on Nov. 11, 2011. While I was photographing these birds, I assumed the darker first-cycle gulls were Herring Gulls (Larus argentatus smithsonianus), but inspecting the photographs I could not confirm any 1st-cycle Herring Gulls, and just about all of them appeared to be THGU. I differentiate the 10 juvs with letters. Identifying THGU in Colorado is always tricky, and this gallery hopefully will help some folks gain confidence with this identification challenge.

There were also a pair of adult Lesser Black-backed Gulls (Larus fuscus graellsi, photos at the bottom of the gallery), dozens of Herring Gulls, and hundreds of Ring-billed and California Gulls. About half the flock was floating out over the reservoir, so there may have been even more THGUs present.

Caveat- As I mentioned, gull identification can be tricky. If you feel that I have erred in the identifications, please contact me privately or post a comment in this gallery. Please sign your messages or comments.

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