I think if it was something big, you'd have more than one dead. Since you mentioned north Alabama area, it won't be a cougar or bear anyway. If the tear is the access point, it would have to be something that can climb that cloth. I'm would guess coon or weasel in your area but they tend to make a horrible mess if they get in and kill/maim almost more than they eat. Possibly a cat, but that would be one determined cat.
Is it possible the duck died and something fed on it post mortum? This is a puzzling mystery so please keep us posted if you discover the perpetrator. It may help someone else also.