This is a female tufted duck.
I find female tufted are very variable and can be confusing, often looking a bit like scaup. The white band around the bill is a fairly common feature in tufted and is not exclusive to scaup, not an exclusive identifying feature. It is typically larger and rounder on scaup.
Female scaup would look bigger, rounder-headed (no crest), broader and longer-bodied and heavier-billed. Also the bill is mostly grey with very little black on the tip (not in a broad band around the tip as in these photos)
There was recently a very good photo posted on the forum of a pair of scaup in flight to compare with this bird.
