I have two barred rocks that I acquired as adults and added to my flock, at that time mostly RIR's. They held there own with the aggressive RIR hens. One is lower in the group, but I think she is just used to being there, she is small and mostly bald on the back of her head, and was like that when I got her. The other one is one of the highest ranking hens now, she picked her place in the flock and stuck to it. She actually fought back like a rooster when the resident hens tried to put her in her place. She wouldn't even let the rooster near her. They are the first ones to take a peck at the other birds. And they don't just peck, they hang on until their point is made! It has never become a problem though, the others just stay out of their way. The higher ranking barre rock was the first to challenge my new little bantam hen. I put her in at night, and in the morning she had stayed on the roost until I let them out. When she finally flew down and came out it was that BR just jumped on her. She took it a couple times and then she'd apparently had enough and ran off, but she didn't just go hide, she shook it off, and then came FLYING back at that BR feet first! They had a little tussle, and it ended with both of them hopping around facing each other off, and apparently they decided it was a stalemate. That was the end of it, but only because the bantam fought back. I think she would have kept at it if it was a hen that wont fight back.
With that said, I also have to mention that when I introduced my chicks to the flock early this summer, I sectioned off part of the coop for them, with their own access door and run, parallel to the big run. When I felt it was time, I took off the wire over the top of the partition, off leaving just the 2ft high solid wood partition. The barred rocks were the first to breach it and would go out in the chicks run and never bothered the chicks at all. I never saw one touch a single chick. Now that the chicks have grown and are about to start laying it's a different story. They have to watch out now.
I dont think they are more agressive persay, just very habitual in nature. I think when they moved in with my birds they just took the same place in the order they were in where they came from. I think they dont have a problem unless that order is messed with, and they didn't see the chicks as a threat to that order until now. I think to them that order is very important, it's LAW, and if you dont bother them they wont bother you.
Of all my chickens my australorp and my marans are the most laid back. They dont bother anybody and nobody, and nothing bothers them.
