Hunters have offered or donated a number of odd Mallards. We can't use these for the hybrid study, but they are valuable for illustrating plumage variants.

Top: Female domestic variant shot by Dave Horvath in Indiana.
Middle: Grey-breasted male shot by Corey Frentress in Texas.
Bottom: young male shot by Glenn Bass in Louisiana.  Young males sometimes replace just their head feathers and retain the hen-like, juvenile body plumage until conditions are better for molting.

These wings illustrate the range of variation seen in Mallard wings submitted to the USFWS harvest surveys each year.  Most of this variation likely comes from domestic Mallard strains.

Blue-billed Mallard.  This photo shows a Mallard with a blue bill, a variant that crops up frequently enough that taxidermists think these blue bills derive from Northern Pintail genes. Apart from the bill these birds look like normal Mallards. Tissues from these specimens might help find genes controlling bill color, so more examples would be welcome.

Right: Normal Mallard with green bill
Center: blue-billed Mallard from Joshua Osborne
Left: Mallard x Pintail hybrid. These have blue bills.