Eurasian Wigeon, at Dodge Pond, Rt. 1, Hampton Falls, NH, I photographed from very far away. This is an Old World species that regularly shows up in winter in North America, mainly on the coasts but it can appear anywhere. So check ducks you see.
Here's a male American Wigeon, a fairly common North American duck. He has a creamy whitish head and crown with a green swath from eye to nape. In sunlight the green looks iridescent.
We, last week, saw the Eurasian Wigeon at Hampton Falls, NH. It was still being seen as of yesterday. The male is easy to distinguish from the male American Wigeon, but the females are harder to tell apart. The female Eurasian Wigeon, is warmer brown overall, especially on the head, (although head can be grayish like American Wigeon) has gray axillaries, seen in flight and does not have a black line around bill base. The female American Wigeon has white axillaries, is more grayish overall, has head contrasting with breast and flanks and usually (not always) has thin black line around bill base.
See page 20 and 21, The Stokes Field Guide to the Birds of North America for more photos and detailed description of both species.