If you hunt waterfowl you might get asked to participate in a parts collection program where you send in a wing of every duck you shoot. After the season they will send you a summary, this year it had one surprise, saying I shot a "domestic" mallard . I dug up the photo from that day, I'm guessing it was the middle duck that got that description. To me it just looks like an early season bird, certainly not obviously bigger than the other two. My only guess is the small amount of white on the leading edge of the wing might indicate some domestic duck genes?