I've seen several male Eurasian Wigeon that had a green patch behind the
eye, but otherwise looked like pure Eurasians. (Remember, male Eurasians
have a gray body, and a Americans a brown body.) "Waterfowl" by Made and
Burn state that in male Eurasians there's "usually a green patch behind the
eye." I'm not sure I would agree with "usually" (at least for the Eurasians
we have here; this variation may be more common in the European population
of Eurasian Wigeon), but I think this green eye patch is within the realm of
plumage variation in male Eurasian Wigeon.

Mike Donahue
Seattle