Introducing new chicks v.s. Introducing when the chicks are pullets/ older?

If your coop is big enough, a lot of people put their brooder right inside. That way they can start getting used to each other right off the bat. Then when the chicks have gotten their bearings, you can prop the door of the brooder open just far enough for the chicks to get in/out but not the adults.

Having the chicks raised by a broody hen is great because she will tend to defend them against the other adults, but first you have to get her to accept them as hers. Usually best accomplished around three weeks after she starts going broody (she thinks she hatched them). You can sneak day old chicks under her during the night and cross your fingers until morning.

 

I learned an awful lot integrating baby chicks with an adult flock over the years, and I came to the final conclusion that chicks are better off brooded alongside the adult chickens right from the day you bring them home.

As Jen above alluded to, for the safest integration, you should provide a safe place for the chicks to retreat to. I call this the "panic room" method. I employ small chick-size pop holes all over the run so the chicks can always find their way back into their safe pen easily.

I wrote a complete article about all the reasons why chicks benefit from being raised outdoors in the coop or run. It's got photos of my set-up that may be helpful to you in designing your own. The article is the second one of the three linked below this post.

 

It's best to allow a hen to hatch and raise them within the flock or to introduce brooded chicks at 6-10 weeks through the use of a separation pen. It's normal to adult birds for chicks to show up in a flock, and they accept them fairly easy in a week or two. Older birds can be viewed as intruders to be driven off and can be harder to integrate.