Richard-Woodlandwoman has a good idea, cover the run with plastic. Clear would be the best choice in that they would get natural light and have a larger coop. Put down shavings or leaves in the run and use this in the compost pile in the spring.
Chickens can get frostbite, yes. But frostbite is a combination of cold + moisture, and if your coop is well ventilated and has dry bedding, they will not. Contrary to most of this thread, I would not get a heater for my coop. They are fire hazards, and chickens are actually much happier in cold weather than warm weather.Can chickens get frost bite. Its getting cold and how do you get all your chickens to get along.