I have a second, smaller pen adjacent to the large chicken run. My young birds go in this small pen when they outgrow the brooder - at about four to six weeks, depending on weather. They need to be feathered and big enough not to look like baby birds, so my cats won't think they are prey. The big birds can see them and get used to being around them, but not able to touch or peck them. The littles have a small, secure coop of their own. They remain in this arrangement for several weeks, until they are about 12 weeks old. Then I just open the gate that separates them and they begin to mingle without either group really even noticing that's what's happening. A week or two later I start encouraging the littles to roost in the big coop, and after that I close up the little coop. Bingo, done! Soon the littles will be ready to lay and I have them where I want them. I'm sure others do it differently but this multi-step approach works for us and doesn't seem to stress the birds.