aggression

[duck mom newbie]

Hi, I have 10 ducks I got at an auction. One is a Pekin Drake, he chases 2 of the runner ducks and has grabbed them by the back of the neck and pulled feathers out. Is this a mating thing? or is he being a bully, and I need to separate them? I am not really sure if the runner ducks are hens or drakes.
Thanks ahead of time.