The birds is superficially similar to American, with its all-dark looking tail, just showing very narrow at base of rectrices. But - as in many other cases - the rump pattern does not really fit the broadly dark-barred of smithsonianus, leaving only aa very faint contrast to the darker mantle in smithsinoanus. In this bird, the contrast is too obvious to fit smithsonianus, albeit probably within the range of paler smithsonianus (especially American West Coast birds). However, the shown tail-pattern in seemingly regular in North Atlantic Herring Gulls (argenteus), see e.g. illustraion of a juvenile from Iceland in "Gulls of Europe, Asia and North America".
The tail and the outer primaries looks too blackish for a Thayer´s Gull, which should be paler brownish-tinged here.