Mallard x Spot-billed Duck Anas platyrhynchos x Anas poecilorhyncha
I'd seen pure Spot-billed Ducks at this collection before and carelessly assumed these were the same birds - it was only when I got home and looked at the photos that I realised something wasn't quite right. For the male the extensive yellow on one bill, the curly tail feathers and the faint reddish wash on the breast point to Mallard involvement and I think the female is a hybrid too.
A subsequent visit produced a more Mallard-like bird, but this resembles other published photos of Mallard x Spot-billed Duck hybrids. It seems likely that the original birds were backcrossed with Spot-billed Duck rather than, as I had first assumed, the new bird being backcrossed with Mallard. Thanks to Joern for his input on this.
When I first saw this bird I thought it was likely a young male Mallard showing remnants of juvenile plumage (and bare parts), but when I discovered it (or a very similar bird) six weeks later looking hardly any different I thought a different explanation was required. By this time I had established that the above hybrids were in the collection again (the collection has been recently re-stocked after being destroyed) and the most satisfactory answer seems to be that this is a Spotbill hybrid backcrossed with Mallard.

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