Most of the ducks were too far offshore for good photographs, but two birds at Sayer's Park were nice and close. One was an adult female White-winged Scoter, and the other a juvenile Surf Scoter.
I thought the above photographs made for a nice comparison. The White-winged Scoter is noticeably larger, with a round head and a gnarly bill shape, while the Surf Scoter is smaller with a flat head and a shorter, more uniform bill shape. The adult males are unmistakable but the juveniles and females require a bit more study. The Surf Scoter shown above has the dark iris and pale lower breast/belly of a juvenile.
White-winged Scoter is one of those birds that has a palearctic population that is sometimes considered a distinct species, and sometimes considered a different subspecies. Either way, I like their name for it better than ours: "Velvet Scoter".