Babies must be introduced. Usually you section off part of your run or put your new babies in a cage inside the run with the bigger/older birds so they can get accustomed to each other safely. After a couple of days, you can usually let them out and other than some "pecking order" scuffles, all should be well. My problem right now is a VERY jealous big Lavender Guinea. She is in the pen with our Buff Orp Rooster and won't let us put the new Orp girls in the pen. We are going to have to introduce the same way very soon and maybe this Guinea may end up in the turkey pen where she won't be the "boss" anymore..hahahaha.