On this gull, note the overall "frosty-ness" of the browns throughout the bird. Thayer's Gulls often have a more-round looking head, than Herring Gulls, and have a more slender-looking bill. Juvenile Thayer's Gulls have very pale brown feathers, often showing a scalloped-in-silver appearance. The tertials on a Thayer's Gull are about as pale as their backsides (when viewing a perched bird). Juvenile Herring Gulls will have tertials that are slightly darker than their backside. Overall, juvenile Herring Gulls are very dark in appearance, whereas juvenile Thayer's Gulls have more of that from-the-arctic appearance, since that is where they are from :)