a deep bed of shavings should take care of that. as the droppings decompose in the shavings it emits heat ......like a compost pile works .... just top off the old bedding with new so they're not standing in their manure .... don't remove the old bedding - or only replace 1/4 of it when you put new in.
This is what we did when i was growing up in New England....COLD winters there!
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First you need to make sure they can stay dry. A dry chicken can retain its body heat with the insulating properties of its down feathers. Wet down feathers won't insulate.