Domestic Mallard or hybrid

I'm sure this must be some kind of Mallard hybrid, judging by it's greenish head, greenish-yellow bill, and curly tail feathers. It looked black & white under an overcast sky, but showed up brown to the camera. It was the only one like it among many other Mallards and some Green-winged, Blue-winged and Cinnamon Teals. What could a Mallard have mated with to end up with a duck like this?