Here's a Fulvous Whistling-Duck from Delta Lake for comparison. Not the concolored nature of the head, breast, sides and scalloping. This bird also lacks an eye ring.
And several from Estero Llano Grande State Park.
Hopefully tomorrow I will get some flight shots and maybe a view of the feet.
There's nothing online that I can find to indicate this combination of hybrid, Black-bellied Whistling-Duck x Fulvous Whistling-Duck, has been documented in the past.
The following day I was fortunate to get a few flight shots.
Apparently there is a reference of this hybrid pairing from a 1958 book by A.P. Gray, "Bird Hybrids: A Checklist with Bibliography". But nothing since then.