They are not mean, the older chicks will always pick on the younger/strange ones by instinct. It's a process to integrate them together. I'm starting the integration process today. I put my 4-8 week old babies in the coop with the 10 week olds. I kept the babies in the brooder(xl wire dog kennel). If I threw them together, the 10 week olds would kill the babies. I just asked in the raising chick forums how lOng will this take. I really don't know, but I do know that you can nit just throw chicks together. It doesnt work out that way.