We deal with winterkill on a property I hunt at, and I'd like to suggest that the rabbits likely starved, not froze. Cold isn't tough on rabbits unless it gets absurdly cold...but the lack of food will kill them swiftly. We always see a lot of winterkilled deer and rabbits and all manner of other critters when the winter is bad plus if the beavers dam up the river...they dam it up, water level rises, kills all the young growth trees and then bam, that winter, lots of carcasses (plus a tonne of dead fish when the salmon hit the dam and can get no further to spawn!) We solve it by tearing the dam down and hiring someone to trap out the beavers.
When seeing a carcass in the cold, it likely didn't have enough food to power it's body, and without food they cannot regulate their body temperature. Did you know that almost eighty per cent of the food you eat is burned for warmth and energy?
Mammals need a lot of fuel compared to, say, reptiles.
I'm in Michigan, I've seen a lot of "cold advisory" for livestock warnings, but also remember that cattle and things out in pens or pastures are not overly protected from the cold or wind, and that they need to be brought in or they may freeze, especially since they often have dampness to them due to falling snow and slogging through more snow.
So long as your rabbits are "winterized" I would not worry about keeping them outside.
Just make sure they have a good hidebox and lots of straw, as Brittany said. On the very coldest of nights, I used to go throw an old comforter over the outdoor hutch, it held in a bit more of the warmth and kept more wind out of the hutch. Also, a lot more food if it is cold, as they need more calories to stay warm.
I do keep my buns in the basement, but this is not for temperature in winter...my basement is old and cold, the boarded up coal chute still dramatically affects the temperature. Right now it is about 15 to 20 degrees colder than the house proper...I get chilly if I just walk downstairs.
I keep them there for two reasons...one is the neighbors have a vicious, destructive dog who would tear open hutches and kill rabbits...can't tell you how many times he's attacked our pets in our own yard, and twice he's gone after ME. We've called the cops and animal control a few dozen times this past year, two to three times a month, they won't do anything.
I also keep my bunnies in the basement because I worry about summer heat...some days get hot here, up to 95 degrees, but my basement stays around 65 to 70 in summer, plus no direct sunlight.
Best of luck with your buns...if you absolutely must keep them in the basement, see if there is a drain or sump hole somewhere in the basement. Buy a dehumidifier, and cut a hole in the bottom of the water container, and place it right above the drain. That way it will never shut off because it will be empty forever.
A small fan in each corner will circulate the air and help clear up the moisture in no time. I don't have the humidity problem but my dad does at his house, we found that to work wonderfully! Best of luck!
