There are several things you could (and should) do to help your chickens stay warm in the winter.
1. Make sure the coop is not windy. Feathers are a chickens natural insulator trapping hot air that was warmed by the body. Wind ruffling the feathers, will not only ruin the feathers insulating properties, it will also chill the birds.
2. Make sure the coop has ample ventilation. The vent openings should be high up so wind won't blow on them. Moisture and cold air combined are very dangerous, it will cause the chickens to loose their body heat.
3. Make sure the chickens have access to high quality feed, especially before they go to sleep. A crop full of nutritious, high calorie food (such as corn, soy, hard boiled eggs etc.) will help them maintain their body heat in the cold nights.
4. Add warming herbs and spices to their diet. Such as: black pepper, cloves, cardamom, ginger, cinnamon, garlic, ginger, cumin, cayenne chilies, turmeric ans sweet basil. Just a word of warning, if your hens are laying, go easy on the sharp tasting spices, the chickens won't mind it if will affect the egg flavor. Btw, feeding egg-layers turmeric will make the yolks a bright yellow.