I have three 4-month-old chicks and six 3-month-old chicks. They all share a large coop with an attached, enclosed run. I introduced the newbies by keeping them in a smaller pen inside the coop for two weeks. The first couple of days that I let them out with the three bigs were brutal, but they get along pretty well now, four weeks later. Everyone roosts together at night. The problem is that the three bigger chickens won't let the others out of the coop. If they step one foot into the run, it's an immediate attack until they high tail it back into the coop. Two of the three bigger birds are roosters, and one in particular is the one who keeps the younger chicks inside the coop. The other rooster has no interest in leading the younger ones, though he does seem to be the boss of the rooster who does control the chicks. For now, I put the younger chicks in a tractor to give them some freedom. I keep thinking that in time, they'll let the others out into the run. But it's been a month, and I'm still waiting! Is this just part of normal adjustment? The three older ones had been established in the coop for about three weeks before I began introducing the younger ones.