Had 24 brood-mates , 4 different breeds. Never saw it. They were all raised in same box, but I had to add 2 more boxes as they grew since my coop was not done yet. I did notice a RIR being bullied by two golden comets in one of the boxes, so I replaced the RIR with another GC and that fixed it. After they got into their coop, I kept them confined for another week while I finished my pen. After that, I noticed a runt Barred Rock who was limping. It never has gotten better, but I see no consistent bullying. No blood, so I thought it was a mid air collision or something. The runt does stay by herself a lot, but a part of that is not wanting to be knocked down while scrambling for treats. She has learned to go into the coop a bit early to get her spot on the roost. (I always carry her special treats and pick her up to feed her in front of the others) Twice I knocked a golden comet off of the roost and flipped off the light for bullying her as they settled in for the night. Seemed to stop it. I see them mixed everywhere a lot it seems. They sometimes cluster a bit, but not a solid and constant thing.