I have a flock of 9 healthy chickens, and my wife wants to add few more chicks. So, how old should the new chick be before I introduce him to the flock and leave him the flock and coop?
I would just like to add that raising a single chick on it's own and then trying to introduce it to an established flock is a recipe for disaster. Firstly, the chick will be lonely with no play mates which can lead to stress and depression and even health issues as a result and secondly, it will get badly picked on when you try to introduce it to the adult birds and have no siblings for support or to disperse the flack.
I would suggest at least 3 chicks, so that even if one dies, there are still two. If the coop is not big enough to brood in there, you may need to wait until the chicks are near adult size before you integrate and they would need to be kept within sight of the main flock for a week or so before putting them in together.
Alternatively, you could hang on, in the hope that one of your hens goes broody and then give her some hatching eggs. That way, she does all the work regarding incubating and looking after the chicks once they hatch, and because they have been raised within the flock there are no integration issues. Of course there is no guarantee that any of your hens will go broody but it's also probably extremely likely that one will go broody the day after you buy some chicks!, That's broody hens for you! They work to their own timetable, sometimes just to spite us I think.... You gotta love them though!
I had mine in my coop with poultry wire so they could see them and get too know them. I was able too put mine in with the chickens at two months. Most people say three months is the best age because they don't want to fight so they will back down to the full grown chickens. Which means no fighting and the big chickens also act as guides and show the other chickens around if they free range.
I kept the littles in the dane size kennel on the patio during the nights and in a brooder in the yard during the day when it was nice enough. At 5 weeks, they moved from the patio brooder at night to in the coop at night with a fence down the middle. Then graduated to having the bird pen during the day. I let the chicken and ducks in supervised and there have been no issues. The Hazel chicken did peck each little one when they tried to share food, but other than that, she's not cared about them and the ducks are afraid of them.
They will keep getting supervised time with the big birds until I'm confident they will mix fine without my supervision.