Is the duck a drake or a hen? I'm guessing that your roo, or your duck, if it is a drake is the instigator, but if one bird is singled out for abuse, the rest of the flock will often join in. Especially in a crowding situation. While technically, your coop is big enough for up to 14 chickens, your run is only big enough for 10. And when ever you add an other species, or a roo to the mix, you need much more room than that, IMO. The fact that you have an abused bird says they need more space. How much cracked corn are you giving them on a daily or weekly basis? They may need more protein. Even if they were just getting layer, without the added cracked corn, they may be deficient in protein. That can lead to feather picking or aggression issues. If you have a drake in that coop/run, that can be a serious problem, as a drake who tries to mate a chicken can kill her.