We have two duck sisters that are currently laying eggs and generally are side by side at all times. We've seen them mount one another in the pool and bite each other's neck while submerging the other. They both do it to each other often. But today one of the girls neck is bloody and missing feathers! We just introduced a new chicken to our chicken flock, would this cause her to be overly aggressive? Should we separate them? Any advice would be appreciated, thanks.