Each bird can and will keep themselves warm. Think of being at the football game, it is your coat that keeps you warm, not the person next to you unless, they are blocking the wind. Birds need to be out of the draft, but not locked up tight, trapping heat in, moisture is what is really trapped in.
What one needs to worry about and address with winter is keeping birds dry. Dry chickens are warm chickens. Too small of coops, not enough ventilation, moisture condenses and rains back down on the birds. Wet damp chickens are cold chickens.
Good ventilation, dry bedding and enough space are the most important aspects to chickens in the winter.

