Just other possibilities, according to Dave Holderread's excellent book, "Raising the Home Duck Flock." Mallard drakes have two very distinctive curled tail feathers, and the reference I gave you from Dave's web page shows a beautiful picture. And since domesticated ducks (except for the Muscovy, which is another species) were bred from mallards, drakes of domesticated breeds (except for the Muscovy) also have this trait, even ones with all white plumage or plumage that is the same color in both sexes.