looks like a teal that's just a little later coming into its full breeding plumage to me.
I don't get what you mean. Coming into breeding plumage from...what other plumage? It's not coming out of eclipse, as that isn't eclipse plumage. It's not any normal plumage for teal to have that wide vermiculation pattern. Structure does look a bit off to me too - more square-headed - but that's maybe just posture. It might just be an unusually-patterned Teal, but it doesn't look like a 'normal' Teal.
If it isn't eclipse plumage it's coming out of then it's a first winter bird, are you honestly saying you've not seen teal looking like this? There are tons of them looking like this in the Autumn.
Here's a link:
http://www.birds.cornell.edu/crows/domducks.htm
Now I'm getting into this more, yes, Poecile, the vermiculations don't correspond with what you'd expect at first glance, but bear in mind that these old feathers will be worn and tatty and are already translucent, these transitional plumages go through many degrees. Don't think a hybrid theory sits comfortably as I can't see any other species in it.
http://www.pbase.com/image/77379631
http://www.pbase.com/image/77379633
I'm afraid I'm not fluent in fancy words to describe its plumage, but I just see a teal here.:t:
This has nothing to do with eclipse plumage...
The strange pattern of the flank feathers is something you sometimes see in intersexes of duck species of genus Anas.
I think it is in between the pattern of the female and the fine black and white vermiculations the males have on their flanks.
So this could be an intersex Common teal;
however, the head shape of your bird also looks slightly odd...before I give a final vote i´d like to see more photos.
What I mean is what I try to show with the attachment , this pattern not occurring in all intersex mallards or pintals though...