Brrr.... 35 degrees here as I type at 4:15 AM. Actually I look forward to this time of year; fall has always been my favorite season, and I love writing here in the front room next to the wood stove. I wonder what kind of winter we'll have this year? Last winter we had very little snow, although there was plenty of cold weather. At least I don't worry any more about how the birds will handle the cold; as David says, that's why they wear those nice little down jackets.
I do remember, though, when I was first researching chicken breeds, trying to choose types that would do well up here in the mountains. All the charts said "cold-hardy," or "not very cold-hardy," but I didn't know what exactly that meant. Would I need to heat the coop somehow when the temperature dropped below, say, 20F? I couldn't find any information or suggestions that were any more specific.
Probably the most important thing I learned (eventually) is how chickens roost; surprisingly, the width or diameter of the roost itself makes a difference. It needs to be large enough that the birds' feet don't wrap all the way around it. When they settle down on the roost, their feet are covered up by their feathers, and if their little toes go all the way under the roost, they won't be under that toasty down blanket. If it's cold enough, this can result in frostbite. So, for chickens, we use nothing smaller than 2x2s for roosts; for turkeys, a 2x4 with the wide side up seems to work well.
The other consideration for chickens is that roosters, when they sleep, do not tuck their heads under their wings like hens do. This can leave their combs vulnerable to frostbite, especially if the breed is one with a large single comb. Fortunately, we've had no problems with that, although the past two winters we've had our share of single-digit temperatures.
Ahhh, a nice cup of tea by the wood stove. It's wonderfully cozy, and life is good here on the farm.