When separating a female Common Eider from an American Black Duck, what structural difference is noted?

Answer

The female Common Eider appears bulkier and heavier.

Although both species show barring, the female Common Eider has a heavier body structure compared to the American Black Duck. Furthermore, the eider's barring tends to be more uniform, and it possesses a more clearly defined pale facial patch.

When separating a female Common Eider from an American Black Duck, what structural difference is noted?
birdidentificationduckEider