I have a Rouen (M) and Khaki Campbell (F) and they are about 6-8 months old. I just recently got a new duck who is a one year old female Indian Runner. The Khaki is mean to her! Both are female and yet, she beats up on the new duck! I fear for the new duck because she is smaller and won't defend herself. What is this behavior that is going on? Or are they "girl-fighting"? Did I introduce her to them wrong?
Introducing an adult duck to existing ones.
Ducks are often very aggressive toward new ducks, this is why when getting new ducks you need to have them in separate pens for awhile so they can get used to each other. They should be separated by a fence at all times so they can all see/hear each other, but no one can get hurt or picked on. Are you able to separate your new duck from your older ones for now? Sometimes it takes awhile (weeks, maybe longer) for ducks to get used to each other and live happily together. It's just the way ducks are.