Hooded Merganser, a rare transatlantic migrant in UK.
Above is the drake in breeding plumage with his amazing crest up which makes his head in side-view a white dome bordered in black; head-on, he has a girl's white wig and a black top-knot.
More of the amazing crest.
Above is the drake with his crest down
and raised again.
The drake has a yellow eye
Hooded Merganser pair
The female Hooded Merganser has a grey-brown body and ginger "hair". Her eye is reddish brown and bill edged with yellow.
She can also raise her crest
The drake's non-breeding (eclipse) plumage is similar to the female plumage; above photos show him on the turn between outfits.