Hi Thomas!
I'll start with a disclaimer: I'm not an expert on Mexican Ducks.
The reason I'm putting in a word is because I remember learning from a waterfowl presentation that Mallard ducks (both sexes) and Northern Shovelers are the only dabbling ducks that have white tail feathers. Since Mexican Duck is not either Mallard or Northern Shoveler, a pure Mexican Duck should not have white tail feathers. I do agree that this bird looks to have Mexican genes. The bill and head look consistent with Mexican, but the tail looks more Mallard to me. I'd suggest this is a Mexican x Mallard, although I'm eagerly awaiting the experts to weigh in on this one. Great photo!
The birds are happy, and so am I
~Caleb Alons, Larimer County