Most of us here will
quarantine any new birds in a place away from the rest of the flock for the better part of a month before integrating with the flock. This accomplishes several things. The most important of which is that the new birds have immunities to things in the environment from which they CAME. The existing birds have immunities to the current environment. These may not be the same, so each group has the potential to cause illness in the others. The typical looks like this: -
2 Weeks totally separate -Then get
1 bird from existing flock and add to new birds. That way if there is an illness you only lose one of your existing flock and you can see how the new birds do.
About 2 weeks like this. -Then if all goes well, move them to an area in which they can all see each other but not get at each other. Give that
another week or so. -Then remove the division and see how they do together. This is way better done if they have a large area they are running in outside so there's lots of space.
That was the "Reader's Digest" version.