Common Goldeneye (left) and Barrow’s Goldeneye (right) (Bucephala clangula and Bucephala islandica)

Not the best photo but it clearly illustrates the field marks/differences of these often confused species. Note the shape of the face spot (elongated in the Barrow’s), the amount of black on the back (Barrow’s has more black), and the sheen color of the head (Barrow’s is commonly purple instead of green for the Common)