|First cycle Thayer's Gull. North Point Marina Illinois. 22 January 2012.|
I consider this individual to be on the dark side for Thayer's. Here's this same photo lightened up:
One mark that I like to use in flight - although not popularized in our field guides - are the undertail coverts. Thayer's tends to display more neatly patterened undertail coverts that show horizontal bars:
|Same individual as above.|
Similar aged Herring's tend to show undertail coverts that are more wavy and "messy". Often times, the buff coloring is thiner (or thicker) in width than the alternating brown rows in Herring.