The rabbits, if they are not a dwarf breed, should be fine in the cold this winter without heat, providing they are sheltered from the wind and snow. Straw holds in the heat better than hay, and is very absorbent and easy to find in gardening stores this time of year. Hay will also need to be provided because straw is not good feed. Rabbits love to graze on grass, but make sure the grass is dry to help prevent parasites. My rabbits have an outdoor run, and I give them a few pumpkin seeds daily, sold a squirrel food, because pumpkin seeds are a natural dewormer.