I've got six chickens that are about 18 months old. Had them since chicks. This spring, we bought six more chicks and combined them when the the chicks were about three quarter sized adults. After three months, they still run around together in two separate flocks and the older chickens, one in particular, still peck and harass the younger birds. The chickens are all about the same size now, but the younger ones don't defend themselves. One funny thing is that in the younger set I have an Australorpe rooster (by accident). He STILL runs and cowers when the older hens come after him. Heck, I thought he'd be in charge, but he gets pecked just like the others. There is one clear leader out of the older group, and she takes after all the younger ones. The run size is about 6x25' so I'd think they'd have room. Will they ever integrate? Or does it even matter? I don't want the older ones interfering with the young ones when they start laying in a few weeks.
I'd put your lead hen on lockdown for a week....this can change your flock matrix and your young roo just might get some roo testes and take charge. When your lead hen comes back she will be the outsider and will have to work her way back into the new pecking order.
My main flock is 2 weeks older than the 3 younger, and 2 are a month younger than the main flock. The 5 younger stick together. I integrated them when they were 5, 7 & 9 weeks old, they were separated by fencing outside for 2 weeks & brought in at night. But then they were getting too big for the brooder. I put the brooder in the coop for two weeks, with the lid on. Then on week 2 let them out, the younger 5 roosted on the top shelf, while the main flock still puppy piled on the floor in the corner. Too the older 9 almost a month to move up to the roost. But my 5 still huddle together in a corner at night. My BO will reach over and peck a head from time to time. Even when they are let out to free range, the younger 5 still stick together, so I make sure they have separate food & water in the coop, if any of the olders are being mean. They have been all together since July 4th, and there are still issues. They may always have issues. Am waiting to see if my roo, from the main flock will mate with any of the youngers when they mature. Good Luck!
Sometimes they will, sometimes they will always be separate. Shoot, I've had the same thing happen when all the chicks were purchased together *shrug* Try removing the head hen and see if the rooster steps up. If he does, he will keep them all a bit closer together when they are free-ranging. If not, is it really a big deal? In a large run, just put out a second feeder and some more water. I try to always keep at least 2 water sources anyway, just in case one gets knocked over while I'm gone
