Recently, I had the good fortune of being able to study Eurasian Wigeons at the Sayville Golf course on Long Island. I had seen male Eurasian Wigeons before, but I was keen to study the females that were at the location since I had never had the opportunity to see them. I took some time and studied both American and Eurasian Wigeons taking photos and making field notes. There are a some field marks that are commonly used in the field to aid in the ID of a female Eurasian Wigeon, but they are not always easy to see and so I decided to do a photo study and share my observations.
First, the difference in head color. If we look at the female American wigeon, it has a starker contrast in its plumage between the neck (gray) and chest (brown); while on the Eurasian its warmer tone of a cinnamon brown color just blends the neck and chest together without the demarcation seen with the gray and brown on an American. A second field mark and boy is this one subtle. The forehead is steeper on a Eurasian and sloped on the American. Finally, the gape (the base of the bill where the two mandibles come together) is black on the American wigeon and there is no black on the Eurasian. It is difficult to see that in these photos. One sure clincher is the gray armpits in Eurasian which is white in an American Wigeon.
Here are some photos showing the comparison.