There is a few variables about frostbite among them exposure-time,temp,humidity/moisture and wind; condition/over all health-level of hydration, proper nutrition and level of stress.
Among these I personally rank proper hydration as the most important by far,as water is critical for every bodily function including maintaining body temp! ICE FREE CLEAN WATER AT ALL TIMES ALWAYS EVERYDAY ! Second would be proper shelter that offers protection from wind/drafts yet allows for proper ventilation, protection from moisture in the forms of rain/snow and condensation again proper ventilation. This means they have access to food and water in a protected enviroment not outside in the wind and snow. Frostbite can occur very quickly to a wet, hungry, thirsty, stressed bird. When the temps dip below freezing give them some extra food, a few handfuls of scratch and maybe a warm treat, like Moms will always tell you "a full belly will keep you warm". Chickens can handle some seriously cold temps well below zero as long as they are well hydrated, well fed, low-stress as possible as cold is stress, dry and sheltered with proper ventilation. From someone who has had a touch of frostbite it stings like all get out so an ounce of prevention, you know the rest. These are just rules I try to fallow....
Good Luck And Remember ICE FREE CLEAN WATER AT ALL TIMES ALWAYS EVERYDAY !
