Having kept a small flock of chickens for 7 years, I've experienced the integration process numerous times and have found the smoothest, most successful integrations to be with a flock of well established mature hens and 3-4 week old chicks (weather permitting). I like the slow-and-steady method by allowing my hens to see the chicks through a barrier for a few days and then allowing short (beginning with just a few minutes) sessions of SUPERVISED mingling, several times each day and increase the time a little more with each session. Inevitably, the hens are more interested in the chicks' feed more than they are the chicks and eventually they're all eating peacefully (for the most part) together and once the hens allow the chicks onto the roosts in the evenings, I feel confident that the chicks are ready to stay in with the big girls permanently. I recently had a lengthy integration with very mature rooster into my flock of hens, having to add 4 new girls in hopes of keeping him occupied. It was a complex situation as I was not only trying to integrate a new rooster who was recovering from what his previous keepers believe to be a bobcat attack, but I was also integrating the new hens in with my established flock. In the end, the rooster went back to his previous home where he was much happier with the considerably greater number of girls at his beck and call and my own girls were able to settle into one flock much more peaceably. His story can be read here: https://www.backyardchickens.com/t/1112288/beginning-a-roosters-road-to-recovery I did not intend on replacing the rooster any time soon but knew that I would want another eventually. The opportunity to add one came much sooner than expected and one of a breed I have long coveted, only recently acquired (amongst the four newest hens) and became immediately enamored with; the Dominique. The new cockerel is 5 months old and I was taken with him the moment I went to pick him up. His keeper walked into the run where he was still amidst his flock mates, walked straight to him and picked him up, handed him to me without any sort of struggle and she told me, "I haven't handled him much so he's a bit flightier than the rest of my birds." After a long car ride home and not wanting to stress him out any further by moving him from his crate to yet another coop, I brought the crate into the kitchen, gave him some food and water and covered him for the night. The following morning, I diced up some tomato, opened the crate door and he very gently took each piece of tomato from my hand. Afterward, he stepped out to look around and I picked him up without a fuss. He nestled into my arms without restraint and looked around curiously as I talked to him and petted him and looked him all over once again. He met Willie, my gentle giant of a Newfoundland and wasn't the least bit bothered. He was used to a small, yippy dog. This went on for close to 45 minutes before I took him outside to the isolation coop. The moment I put him in it, he went frantic trying to get out. I worried that he may injure himself so I let him out and he followed me back to the house. Then he heard the hens and went over to see them. He desperately searched for a way into their run and they were all very curious about him. Typically, there would be weeks of quarantine for new members but seeing how he reacted to being put in his coop alone, I knew he needed companionship right away. I picked him up again and carried him into the run with the hens, squatted down and set him on the ground. My eldest and top hen immediately approached him, raised her hackles at him, they had a few seconds of sparring and that was it. He ran to me and hid behind me. She would periodically approach him throughout the remainder of the day and when she did, he would dig a hole in the dirt and lay in it with his head resting on the ground as she circled him, like a shark circling a dinghy a few times before walking away again. She was letting him know that she's the boss. He was used to some very aggressive ("murderous" was the term his previous keeper used), mature RSL hens who had already whipped him into submission and he had no problem accepting that he has not earned a higher place in my flock. The day went on with the flock pecking around the yard. He had not been able to do that much before as they had a fox den with a whole family in it on their property. He was so happy to be able to run. He kept zooming past the hens and ducking under the shrubs only to dart out again to run past them. He would run up to me and stop a few feet away until I would squat down and then he'd get closer for a rub on the head and neck. Each time he approaches me, I pick him up to hold him and pet him and have my children hold him as well. He really seems to enjoy the affection. As that first evening of him mingling with the girls approached, I planned to put him in isolation, for his own safety. As the girls all filed into the coop to settle in for the night, he hung around the door to the kitchen. Amelia, my Golden Spangled Hamburg, the smallest of the flock, kept rushing up to him, chattering a bit before heading back to the coop. She did this several times and I realized she was trying to get him to follow her but he wouldn't so I headed to the run and once he was in it, I closed the door. Eventually Amelia convinced him to follow her into the coop. I still did not have much hope of him being able to settle in with them so quickly but figured he at least wouldn't be alone until it was dark. When it was time to close up the coop for the night (if I go out too early, they all rush back out) I was thrilled to see him up on the second roost sandwiched between two hens. I gave him a quick pat on the head and locked up for the night. I was up early yesterday morning as I was unsure whether I'd be finding a half dead cockerel in the corner of the coop but to my delight, there he was, standing in the middle of the coop floor, surrounded by all 10 hens, quietly waiting to be let out. A few of the girls will chase him away from the feed so more feeding dishes have been added but he's much happier to be foraging and he always has at least one or two hens around him that lead him back to the rest of the flock when he wanders too far from them. Though giving my last rooster back was not the easiest decision, I now see it was for the best. He was the catalyst that changed my mind about keeping another rooster and though he wasn't the best fit for us (nor us for him), he was a step in finding one who is. I do not recommend such an abrupt integration but this has been by far the quickest, smoothest, simplest most peaceful transition my flock and I have ever experienced and I'm looking forward to seeing the kind of rooster he'll turn out to be.