ID help on a duck - possible hybrid

We have a pond in the back yard here in snowy Massachusetts. Because there's a few square yards of open water where springs feed fresh water into the pond, several dozen mallards have chosen to spend a cold winter here instead of going south. My neighbor and I have been throwing cracked corn into our yards to keep the birds alive this winter. This morning after throwing corn onto the snow and watching the mad feeding frenzy from the comfort of our warm kitchen, our granddaughter noticed that one of the ducks was different from all the other mallards. I got this shot of the interloper before the flock spooked and flew back into the water. Any thoughts on what it is?

My wife read on Wikipedia that mallards have been hybridizing with many other kinds of ducks, and have been known to cross-breed with as many as 60 other species. This one has the bright yellow bill of a male mallard, but none of the other obvious markings Our best guess is a hybrid mallard-American black duck. Any other suggestions?

7D, 100-400L at 400mm, 1/320 sec, f/5.6, ISO 400

Looks like an eclipsed mallard drake transitioning back into breeding colors, or for some reason going back into eclipse now. Could possibly be a hybrid with a black duck.

edit-I googled mallard/black duck hybrid. I'm sure that's what you have based on results. Interesting catch, nice to see a hybrid that's less likely to be a result of human impact-both mallards and black ducks have similar ranges and habits. I've also seen what looked to be mallard/pintail hybrids but long before I was into photography.

Yes, definitely a Mallard - American Black Duck hybrid, saw a ton in Newfoundland on recent trip.