I usually "isolate" the Mama and the babies together after their all hatched out. I usually give it at least 48 hrs. from the time that the first chick hatches. In my experience....babies tend to stay right under Mama for the first few days anyway....so they aren't going to be venturing out from under her anytime soon. And come on...honestly....that Mama hen is NOT going to tolerate any other chickens even coming close to those babies!
