New Duck on the Pond

At first I had no idea what I was seeing other than a suspicion that this was a mongrel/manky mallard.

A mongrel or manky mallard is the result of a cross between wild-type mallards and domesticated ducks (that were originally bred from mallards).
Since domesticated ducks are generally bigger than their wild cousins the mongrels are generally a bit larger as well. That is what puzzled me as this bird was quite a bit smaller than the wild mallards - more the size of a coot.

After a bit of on-line searching and suggestions from contacts, I have concluded that this is either a pure magpie call duck or predominantly such.

Call ducks are new to me. Originally bred as decoys for hunters to use, they are now chiefly bred for show or pets. Many now apparently live as a feral population having been lost, abandoned or escaped.

One breed of call duck is pure white with a short yellow bill and about 3 years ago, a blipper was posting a group of 5 such birds on her local pond. They were not identified at the time (as far as I know) but I now believe them to have been call ducks.