Watch out for frostbite on their combs, and if they get it, treat the area with antibiotic ointment. Chickens are pretty hardy, but nonetheless I have a heat lamp on a timer that I run on the coldest nights for them. They seem to appreciate it. During last year's cold snap I checked on them frequently and they did fine. I do feed them extra high protein snacks and meat scraps during times like this because it takes calories to stay warm!