Just reflecting once again on our excellent recent trip to Arizona and California, and the Salton Sea gulls. Of the twenty or so Thayer's Gulls seen on January 15th and 18th combined, I only detected one adult. This bird, at Red Hill Marina, was conveniently resting next to an adult American Herring Gull and an adult Ring-billed Gull providing an ideal profile comparison, and a good opportunity to compare upperparts tone. Curiously, the mantle shade of the adult Thayer's looked like a close match to the adult American Herring Gull, though I was of the distinct impression that it should (usually) appear darker (ref: Malling Olsen & Larsson and Howell & Dunn). Obviously, not the greatest of images but I couldn't detect any obvious difference in mantle shade in the field, or from the images.
Though only one bird, this impression seems to be in keeping with a discussion on Thayer's Gull mantle shade on the ID Frontiers Listserve. In essence, mantle shade appears to be of very little value as a field identification feature when trying to detect Thayer's amongst American Herrings Gulls, only adding more venom to the challenge of finding an adult Thayer's here in the East!
Thayer's Gull - adult (center bird in all three images) with American Herring and Ring-billed Gulls, Red Hill Marina, Southern Salton Sea, Imperial County, CA. January 18th, 2012.