Speculum, Latin for "mirror", refers to the patch of iridescent color on the secondary feathers (wings) of most duck species.
Both male and female mallards show this feature, edged in white.
On a drab winter's day, the purple-blue-violet color is a welcome sight!
This is an excellent feature to distinguish a mallard from an American black duck in flight. Since both ducks are the same size and shape, it's good to know the black duck's dark blue to purple speculum is without white borders.