i have been leaving my little pop door open at night for the girls to exit in the mornings. it hasnt gotten below 36* here at night yet, so i figure they should be fine for a while. once winter really hits, i will close it up all the way. so my question is, what is the coldest they can tolerate before i shut them in? the coop is not insulated, and the door is a foot square. there are small vents up top of the coop as well.