What I have as shelter for my two chickens is a plastic barrel with wood shavings in it. It's held firmly and doesn't move. Their run is about 5 ft. tall and has no top. There is a tree right above them and at night they fly into the tree for the night. Well, now that it's getting into the 30's at night I fear for them not being warm enough. Why won't they go in their barrel? Won't they freeze?