Your birds will be fine. We routinely get into the -20's here in winter. Some can get a bit shocked from sudden drops, but I personally have never had a problem.
Generally you will see your birds hunch up, maybe stand on one leg, stand close together, and even shiver. It doesn't mean you need extra heat. It's all the ways they warm back up. Adding extra heat will mess with the natural acclamation process where the bird gets used to cooler temperatures, and the coat thickens up.
Nature has provided chickens with a warm coat, and the ability to regulate their own body heat. Your job is to provide a good fresh ration, fresh clean water, good roosts, wind breaks, protection from the elements, and good ventilation, as well as maybe a bit of scratch to get them moving and scratching which also warms them up, and allows you to check that everyone is doing okay.