I have 12 seven week old chickens. I've been using a heat lamp at night but not during the day for a week or so now. Today the temps were upper 20s to low 30s and they seemed just fine - even had them free ranging for about an hour and a half. So with tonight's low being 24 degrees, do you think I can leave the heat off? I really want to get them off the heat but am still nervous. 250w bulb. These are my first chickens.