We had a extremely mild winter here, usually it gets as cold as 40 below, this year I don't think the temp dropped below 0 for more then a day or two. I put an old pickup topper in the colony and stuffed it with hay (mostly alfalfa, and crested wheat grass) after they chewed that up they proceeded to dig a warren under it. I also took a 250gal plastic chemical tote, cut that in half, installed a heat lamp, and cut a small doorway, this is what I kept their water in, which stayed mostly thawed (next year I'll add a little insulation to it). All the rabbits survived the winter just great, even managed to get a few litters to boot. I have found several dead kits pushed out of the hole. I am hoping this is because the mothers are first time moms and inexperienced. Two litters (I think) survived, about 16 little buggers running around. I see some does carrying straw down some holes, so I presume there are more litters around.
I did lose my NZ buck, found him dead one morning (nice day outside) with a little blood on his nose. I let my California buck out of his pen(in the colony) and he seems to have taken to the girls pretty good. I do believe one of the girls is in fact a buck (another NZ) and after watching them fight a little bit, I'm pretty sure that may be how the other buck died.
I have expanded the colony by another 1000 square feet, with plans on doubling the size sometime this summer, perhaps even tripling it. There is a dead spruce tree (laying on its side) in the new expansion. I cut off the branches pointing up, and threw those and some fresh cut cedar boughs over the trunk, creating a new natural looking shelter, this was done in a somewhat open area so should also provide some protection from the darn hawks.
All in all I think the colony has been fairly successful. If it continues to work well, I am going to build another one or two soon, perhaps some different breeds of rabbits for those.