Boston Massachusetts is a great city, and one of the best things about it is you don't need to go far to get some good birds. I was up there this week for a work trip and got out of a meeting just before 3pm. In the winter the sun sets a little after 4pm, so I knew I had very little time. I got my camera from my hotel, jumped in a cab and was at Castle Island in South Boston by 3:25pm. By 3:45 I was on a Barrow's Goldeneye and I did not even know one had been seen there recently. I simply picked a green space on my Map App.
Barrow's Goldeneyes have a crescent in the lore area where a Common Goldeneye has a circle. Also the patterning on it's back is different.
Common Goldeneye on left and Barrow's on the right.