I have 6 girls that are 10 weeks old. A couple weeks ago I put them in the coop. We've been turning a heat lamp on for about 12 hours every time it got below 40. My wife wants to keep doing it, but I think they'll do okay without using it at all. We live in east Tennessee and it's been dipping in the 20s and 30s pretty frequently. Will they be okay without it?