A very interesting bird! I hope Joern Lehmus can weigh in, but my thoughts:
- Its plumage is very similar to the "bibbed" type of domestic Mallard, if anything between a typical male and female of that type. It's hard to believe that that isn't one of its parents - unless the same mutation that causes that colour pattern could spontaneously occur in another species???
- I don't think it can be entirely domestic Mallard, despite how variable they are - "jizz" and proportions just look wrong, too "small" and delicate looking, particularly the thin bill, and that "tuft" at the back of the head (which combined in the second pic make it almost look like a Hooded Merganser, though I'm not seriously suggesting that species is involved!)
- The "tuft" makes me wonder about Tufted Duck parentage, but I've never personally seen a Mallard x Tufted hybrid, and the only photos I have seen of one are these, which don't look structurally much like this bird:
Mallard x Tufted Duck (despite how commonly the species occur together, and how frequently both hybridise with closer relatives, they don't seem very interested in each other!)
- There is a vague resemblance to Eurasian Teal, so I'm leaning towards domestic ("bibbed" type) Mallard x Teal as others have suggested. Though photos of that hybrid don't seem to show such a thin bill either:
Mallard x Eurasian Teal
Fabricius, what was your impression of the size of this bird? Were there other ducks(/coots/gulls/etc) around for comparison?