Mottled Duck (Anas fulvigula) - 01Jun2017 & 21Oct2017 & 14Nov2020

Per Audubon website: "A close relative of the Mallard, the Mottled Duck is the only dabbling duck specialized for nesting in southern marshes, far to the south of most of its relatives. Unlike most waterfowl, Mottled Ducks are almost never seen in large flocks, generally traveling in pairs or small groups. A major threat to their survival is the release of numerous pet Mallards in Florida and elsewhere in southeast; these feral birds interbreed with Mottled Ducks, diluting the wild population of the latter."

Update 14Nov2020:  This Mottled showed up and ticked all the boxes for a pure bird with no Mallard parentage.

Nice buffy highlights, all yellow bill with a gape spot and the black eyeline does not extend to nape.

Tail had buffy tones too.  Mallards have white highlights.

Leading edge to speculum with no white border.

Here is one from the Lower Rio Grande Valley at South Padre Island in Oct 2017.

This particular bird was found in the marsh by itself at Fort Fisher Spit.

Buffy face and markings on body, lack of white tail feathers, clean unmarked bill and spot on gape all point to Mottled Duck with no evidence of hybridization.

Nice clean white underwings.

Blue speculum feathers with no white borders.