Baer's pochards

Probably Australian white-eyes (Aythya australis).

@gentle lemur Are you sure these aren't Baer's Pochards (Aythya baeri)?

@birdsandbats

I would say Aussie White-eye.
From what I remember Baer's have rounder heads which are greenish...

Bronx keeps several Baer's Pochards and I don't know of any US zoos with Australian White-Eyes.

~Thylo

Thanks to everybody! By the last sentence I will tag them as Baer's.

As these look nothing like my memory of Baer's, I did a bit of Googling. 'Google Images' brings up a picture of a male at the Wildfowl Trust, which is identical to those we used to breed 30 years ago from Peakirk stock. I tried to copy the image or a link to it for comparison, but failed. These birds always reminded me of a slightly coarse Ferruginous Duck with a green (almost Mallard green) head. Is there a distinct eclipse I don't remember, or another sub-species...?
But, G-I shows the Aussie Hardhead as having a distinctive parti-coloured bill, so indeed I think these must be Baer's.

Baer's of both sexes have the green sheen on the head (although it is hard to see from some angles and brightest in full plumage drakes) and the ducks do not have a broad paler brown stripe behind the eye, which is characteristic of the Australian white-eye - based on the illustrations in Peter Scott's Key to the Wildfowl of the World.
Photo of a Baer's drake at Chester from 2008. Note the relatively large head and bill and the rounded head shape.

@gentle lemur

Yes absolutely, this is what I remember (from a former life), as the birds we used to breed from the W/T Peakirk stock.

I saw unsigned ducks identical to these at the Toledo Zoo. The nearby keeper told me they were Baer's Pochard.

@ThylacineAlive Sylvan Heights Bird Park has some Australian White-eyed Ducks.