This Eurasian Teal was hanging out a Jackson Bottom Wetlands. In the United States, Eurasian Teal (Anas crecca crecca) is considered a subspecies of Green-winged Teal (Anas crecca carolinensis), but in Europe, they are considered separate species.
Males are pretty easy to differentiate. Eurasian Teal has white horizontal bars along the back and pale edges on the facial markings, while Green-winged Teal has vertical bars near the shoulder and lacks the pale borders on the face. Females are much more similar to each other. Eurasians average paler on the face with more white in the wing, but the two are not readily distinguished in the field.
Personally, I like to consider them separate species, but my motivation is based on my desire to list two species, rather than any scientific evidence.
Ethical Birder 