It is getting really cold. Last night it got below 20 degrees. My chicken coop has a small space at the top on the front with chicken wire to let the air in when it's warm. It's kind of like a chicken window. And I have a tarp to cover it with when it gets really cold. I have 38 birds that roost in the coop. 11 guineas and 27 chickens. When should I cover up their window??