lou salomon
the birdonist
you probably know that some juv argentatus are very dark and resemble smith herrings. i suppose you have such a dark juv male herring gull here (large, strong bill suggests a male). i can see some window but not enough to say if typically strong like in western european HGs or more subdued/venetian blind like in some baltic HG or in smiths. tail pattern also highly variable in HG, especially darker individuals also being prone to show a very extensive one. a close look on both rectrices and utcs would have helped - and i guess you're right about the probability being very low that a yank would have made it cross the pond already. atlantis is a hard to prove bird itself but usually doesn't show an all dark tail.