Tufted Duck (female)

Birdguycrc, I think you're right. I did a search and the Central Park Zoo apparently does have this rare bird. I'm surprised that they'd have such a rare bird in with white-naped cranes though. Those cranes can be aggressive.... Good catch!

Great shots, RiverValleyRunner. This duck is not a female Tufted Duck or a hybrid bird. It is a male Baer's Pochard (Aythya baeri), which is related to the Tufted Duck. It's a critically endangered bird from Southeast Asia, likely less than 300 left in the wild.

Thanks everyone for the help! From the pics on Wiki it really does look like a Tufted Duck

I agree with Saber, a female Tufted Duck.

It's not unusual in zoological parks to find hybrid ducks and this may be one of them. It certainly has some characteristics of a mallard. It's overall too dark though and while that bill has the shape of a mallard's bill it isn't the right color. Maybe a mallard x scaup hybrid? It does have some characteristics of a tufted duck so it could be a mallard x tufted. Tufteds aren't native here, but since this is in a zoo/park it could be at some point in the past that a mallard managed to slip some of its genetics into the family line. They're good at that....grin.