I have 7 hens, 3 months old. The past three nights, I've gone out to close up the coop and some of the girls are roosting in the pen outside. The first two nights, it was two but tonight a third joined them. I'm in central Wisconsin and we're in the single digits now so I'm crawling into the pen every night to get them and put them in the coop. They've all gone in at sundown voluntarily before this. What in the world would make them do this now?