Wow are you sure another duck did that ? Its your pen covered ? It looks like a hawk or an owl may have done that and the duck got lucky. Ive never had an owl attack, but Ive had plenty hawk attacks and they usually tear into the back of the head like that first. Ive had that happen to a baby duck before buts its been years ago and Im not sure how it happened or what I did to treat it, but it did live for a while and just had a bald spot where the wound was.