I saw this bird in wetlands in Spain. It was in a wild bird reserve, with apparently a lot of northern shovelers, common teals, common pochards, red-crested pochards and mallard ducks (according to their info boards). I suppose it is one of those.
Unfortunately we can't see the colours properly and it was taken from a distance with the sun in my eyes so I couldn't tell. It looked very much like a female mallard - and might be one - but to me the bill looks longer than a mallard's. Is it a teal?