Normally, we only have a week, or two, with temps at -22F, usually not one after the other. Last winter was an exception. We had a 2 week stretch of -40F. It was bitter cold. Lots of people lost birds at that low temp and I guess that is why we saw a run on the Cozy Coop Heaters at the local farm store.
Also, when the temps get that low and stay there, it is very difficult to keep fresh water for the birds. So I wonder if many of the lost birds got dehydrated, cold, and then died. I bought a water heater base for my metal waterer. It is supposed to keep the water from freezing down to +13F. Many people have told me that it does better than that, and I hope it does. I did not have any birds last year, but I hope I am prepared for this winter when I have laying hens over winter.