It is easy to identify the male wigeons as their different head color is pronounced. This American Wigeon shows green while the Eurasian Wigeon has a reddish-brown head. (Note 1: The American wigeon is sometimes referred to as a "baldpate," due to the white streak on the crown of its head. I don't know if this applies to the Eurasian or not.)
When I began my quest to learn more about the differences between the female American Wigeon and Eurasian Wigeon, I was really surprised to find this to be a big challenge. The Peterson Field Guide and others, provide minimal information about the differences. Essentially, the descriptions provided were these: Female American Wigeon - Brown with gray head and neck. Belly and forewing whitish; Female Eurasian Wigeon - Similar to American Wigeon, but head often brown and tinged with rust. A defining field mark of the Eurasian Wigeon is the dusky colored "wingpits."
This picture of a female Eurasian Wigeon is probably the best example I have to match the description above. The head does show a bit of rusty color. In case you are wondering, this photo was taken on January 1, 2010, so the rusty color doesn't seem to be associated with breeding. But, what if the rusty color is not present, which is sometimes the case? There must be other field marks that differentiate these two species.
My Internet search turned up some very helpful articles. It seems I am not the only one struggling with this identification. (Note 4: Sources I found helpful are cited at the end of this post.) When flipping through my collection of pictures, I found this shot taken some time back in my Eurasian Wigeons folder. Now, I am having second thoughts. The sources provide info to cause me to re-think my ID on this female. I elaborate below.
This is the bird I found on January 28th at Bowring Park. The additional information provided by ABA and ADFO gave me a few more identifiers to work with. Color: The female American Wigeon will have a cold gray color on its head vs. the warm brown color of the Eurasian. The head of the Eurasian can be much darker than the body. Nevertheless, there is a lot of variation in head color, making it quite complicated to identify the birds with in-between colors. So what else can I look for? Head shape: The Eurasian has a steeper slope from the base of the bill to the top of the head, which happens to be flatter than the American Wigeon. Bill: The American Wigeon often has a black line that runs along base of the bill and a black line that runs along the side of the bill, referred to as a gape line. (Easily seen on the second picture above.) Often is the key word here. It is not always present. However, the gape line never appears on the Eurasian, so the presence of a black gape line indicates an American Wigeon. (Note 5: This dark line at the base of the bill is shown in the field guide but there is no mention of it.)
Neck: There "tends" to be a distinct contrast between the neck and breast of the American Wigeon that is not present on the Eurasian. Weight: The American Wigeon is slightly bigger, weighing one ounce more than the Eurasian. Face: The face of the female Eurasian Wigeon is often plain while the American Wigeon shows more black around and behind the eye. Underwing: This may likely be the most telling field mark. The female American Wigeon shows white axillaries and wingpits. These areas on a female Eurasian show as gray and do not stand out.
So, not being able to see the underwing, what wigeon did I see at Bowring Park. My guess is a Eurasian. This is why I think so: There is a steep slope from the base of the bill to the head, the top of the head is flat, there is no gape line on the bill, there is no black line at the base of the bill , and the face is plain and darker than the rest of the body. Challenges: There is no distinct gray or reddish color on the head; I can't see the neck to see if there is a contrast between the neck and the breast.
The second photo in this post seems to best exemplify all of the field marks of the American Wigeon as shared above, and the fifth photo is the best example of a female Eurasian Wigeon that I have in my photo collection.
The great resources I studied when drafting this post were very helpful and provide a lot more detail than I have. For the interested, visit: http://www.azfo.org/gallery/EUWI_female.html