Pen them separately - make some sort of divider so they can see each other but not get physical - until the new ones are as big as the old ones.

You can expect to see some feathers flying even if you mingle equal-sized birds. It's 100% normal bird behavior for birds that make flocks. They need to establish who is boss.

That is how they are now, smaller ones are screened in a separate enclosure, growing more slowly than they did as chicks. The larger original chickens can see them but can't get at them. I'll give it time, thanks.

I have a system that works fairly well.

I let my chickens run free for a couple hours every afternoon. So I let the young ones out with the older ones. They have more room to move around in while they are working out the pecking order.

There is always some squawking and carrying on, especially when there are young roosters in the mix. I've noticed my main Rooster is relentless against males not of his bloodline.

What was precious was when my broody hen adopted 15 chicks obtained through mail order. She's got 19 now. In a few weeks I will let her out with the little ones.

Heaven help the critter who threatens one of HER babies. If she raises this batch the same way as the last batch, she will bring them in the coop at night. They will be integrated into the mix that way.

I wouldn't worry about a few pecks on the head. A serious fight is crouching and fluffing feathers, flogging and getting on top of the unfortunate, pulling out a lot of feathers.

I've seen hens take on drakes who are twice their size.

Mind you, I have Barred Rocks and Dominques (with a flock of Moscovie Ducks.) The Rocks are more aggressive (and noisy) than the Dominques. (The Rocks are going to market this fall, since I have a supply of Dominques.)

Different breeds have different levels of aggression, so what works with my relatively docile flock won't work with say...Rhode Island Reds for example.

I have the same question.. I have 5 chickens that are fairly easy to get along with and 1 rooster that is a sweetie and lets the ladies run all over him, they are all White Leghorns. The new ones are only 1 month old and there are 5 Black Barred. 5 Rhode Island Reds, which both breeds seem fairly agressive and 4 Arcunas that seem mild.

The older chickens are in an older small building and the new are in a box in our sun room. We are building a new home for them all to share. I was thinking about putting the new one's in first as I want them out of the house, then since they will be out numbered, one by one adding the older hens with Charlie the rooster last. I was going to put Charlie out first, thinking that he might protect the new poults, but then was afraid that he might want to mate with the babes and, well are they too young for that? How does that whole arrangement sound? We originally planned to have an area where they could just see each other for a while, but the more we build this new home for them, the smaller it seems to be to be getting, and it is hard to fit all of our "wonderful" (LOL) ideas such as an automatic feed bin and access to the nest boxes from the inside "people" side of the hen house. They will have a nice size yard, but what about when it is time to go in for the night?

I am very worried about them hurting each other, but geeze, nature must handle it someway.

You might want to check out Velvet Sparrow's Chicken info website. There is plenty about this subject there.

A link that might be useful:

Introducing New Birds To The Flock
jackshenhouse.com/VSChickIntroducingNewBirdsToFlock.htm

I'm going to mix my chickens tomorrow. Was going to do it tonight but time got away from me and I didn't feel up to it, LOL. My layers and meat birds have been together up until now, transplanted to another caged area within the coop building. I plan to put my layer girls in with the big girls after they all go to bed, not sure how that will play out. The meat birds will stay in the area they are currently in and hopefully the decline of their numbers will allow them to grow big more quickly. DH was just saying that the birds don't seem to be growing like they were but I think it is just a natural lull. I didn't get CC meat birds so I try to remind DH that it will take a few more weeks for them to get to butcher size. Lori

I think I've had a mixing of the girls successfully. Last evening DH and I put all the little girls in with the big. The little ones stayed in the coop most of the day, too fearful to venture out until it was getting later and some of the big girls were hanging out in the coop as well. It appears they are taking their licks but I don't see any blatant abuse going on. I wish I would have clipped their wings again but didn't think of it at the time. Lori