I used grass hay. We can get to -16° F here and no one suffered with mites, fleas or frostbite. They made tunnels.
FWIW, I had 14 bunnies each in its own 3'x4'x3' hutch. I had them well over 7 years before giving them to a friend in the country.
FWIW, I had 14 bunnies each in its own 3'x4'x3' hutch. I had them well over 7 years before giving them to a friend in the country.