This article is comprehensive and exhaustive, and it provides the information any chicken keeper is searching for in their first year, but often cannot find on the web due to the many opinions on bird comfort and the wide range of temperatures. I especially applaud the relevant photos and the details on chickens' adaptations to cold. I live in zone 3a and believe in the importance of allowing chickens to rest from egg production and use this energy for feathering and acclimatizing themselves to winter rather than heating and lighting the coop.
