Hi, I have a small back yard flock. All but one of our old flock had died to we purchased new chicks and ducks... they got along fine tell recently. Our older chicken is now attacking my duck standing on her neck and pecking at her head. I’m not sure what to do and I fear for my ducks life, we have her out of the coup and staying indoors so she won’t get hurt. Does anyone know why this is happening and what I can do?