what are they dying of? I assume it isn't weather. I was shocked when my Wisconsin coworkers said their chickens handle -10F weather no problem in the winter.
there's just a pile of things. usually reproductive -- becoming egg bound, etc. there's parasites, viruses, bacterial infections. and being flock animals, they hide it, too.
yeah some varieties of chicken are remarkably tough around cold which is crazy considering their original tropical origin.