Erika and I drove to Prescott, Wisconsin in hopes of finding a Barrow's Goldeneye. This species is a western bird that is occasionally seen in Minnesota. Among males, two field marks usually stand out. The Barrow's Goldeneye has darker sides than do American Goldeneye (the duck expected in midwinter Minnesota). I was excited to see a couple of dark-sided goldeneyes. But these birds both lacked the second field mark--Barrow's Goldeneyes have crescent-shaped, not roundish, face spots. Check out the crescent-shaped face spot on the Barrow's Goldeneye in the last photograph on this post--a bird from Rapid City, South Dakota. I decided that these dark-sided Prescott birds are first-year winter male American Goldleneyes.
A third field mark is that the Barrow's Goldeneye's bill is shorter than the bill of the American Goldeneye. I think the difference in bill shape is fairly clear in these two photographs. Female goldeneyes are generally harder to identify. Note the female American Goldeneye in the middle of the flock above. Its bill is yellow-tipped. The female Barrow's Goldeneye's bill is most often almost all yellow.