Black Scoter Melanitta americana
Formerly thought to be the American subspecies of Common Scoter, this taxon is now widely recognised as being a distinct species.
A rare vagrant in the UK, the only one I've seen was this very distant individual far offshore from Llanfairfechan in North Wales. A more recent individual in my home county of Norfolk sadly didn't cooperate for me.
Black Scoter (second from left, with 4 Common Scoters), Llanfairfechan (Conwy, UK), 20th November 2005