I was worried about that two when I was combining my chicks. Not to mention these are our first chicks ever! I have 3 3+ week olds, 5 2+ week olds, and 2 roo chicks that are bout a week and a half. They are all doing just fine! The older ones fly the coop and are outta the brooder before I know it, and the younger ones do too if can get enough height!