I have four chicks, and they are about five weeks old. We have a fully enclosed building that is the coop, with straw and roosting boxes. During the day I bring them outside as long as it isn't raining. I've been keeping them in a small pen in the garage at night with a heat lamp on. How long do they need the heat lamp for, and at what age are they ready to be in the coop at night and not need a heat lamp? Thank you!