Maybe space is an issue ? This spring We started with four ISA Brown chicken hens and two Mallard hen ducklings, all were babies about a week apart in age. Now that the ISA Browns are getting close to start laying eggs and I see them pick at the other ducks out of the blue and sometimes chase them a bit in the community coop. So I am in the process of moving our ISA hens into their own coop to help avoid any possible issues from being all together. I will still let them all free range together during the day, but at night they will have their own space. IMO, as chicks it was ok to raise them all together for the extra company. Now that they are bigger and the chickens have different needs then the ducks. I think it might help avoid possible future issues being separated. Just my take on what I see when they all interact together.