Dodgy gull

Gulls pose all sorts of identification problems, not least with the proliferation of possible species to identify since my early birding days.
One of those new species to consider is the American Herring Gull, formerly a subspecies. 
Two have recently appeared relatively nearby, one on Lewis and the other in Fortwilliam
This bird came across as really dark and an all balck tail is a good but not infallible indicator of the species. After a few seocnds on the shore, it flew away. Herring Gull it almost certianly is but.....