Given the extreme variability we witness in Kumliens Gulls in Newfoundland, "real" Thayer's Gulls are extremely hard to pin down. I found this bird three days ago feeding at a sewage outflow with a few hundred Kumlien's Gulls. There are Herring Gulls and Greater Black-backed Gulls in the harbour but they rarely visit the sewage outflow. This bird was feeding among Iceland Gulls and feeding in a similar fashion, nervously pecking at the surface of the water. It was often in the middle of the action and aggressive. While generally the challenge here is to determine whether there is Kumlien's influence, the challenge with this bird seems to be differtiation from Herring Gull. Below I have a series of not great photos but hopefully it will be enough for those with extensive experience with Thayer's Gull. There were a few 2nd winter Kumlien's Gulls that looked very similar this bird but the wings were always paler with more extensive pale tips. As well, they tended to have a more short-billed, round headed impression in comparison to this bird, which looks to have a rather sloped forehead.
I appreciate any thoughts on the identification of this bird.