If your does have good instincts and you provide plenty of insulating materials there isn't such a thing as too cold for a rabbit unless you are trying to raise rabbits in the arctic circle. With a little straw and out of the wind my rabbits were active in -30F (before windchill). Most of my does managed to raise litters in that. One doe failed to block up her nest and they froze but she's also failed to raise a single litter outdoors. I'm not sure what it is since she feeds them so well but she's only raised litters successfully in the house. My creme d'argent impressed me in to keeping her despite changing my mind about breeding cremes. In the middle of winter she built a 3' high, 3' wide and 2' deep mound around the back half of her nest box and then closed up the entrance so the kits were at least 6" in from the top wood and front opening with the back, top, and sides buried. In the middle of summer she dug a light nest in the sand pile I gave them. She hasn't lost a kit except to some rats.