It's cold! Record lows in the area have brought our chickens to an indignant resolution to stay in their coops.
In order to manage chickens in colder temperatures one needs to consider several things:
Ventilation- there must be adequate ventilation to allow dissipation of the moisture from the chickens' breath. If not, the moisture will settle on combs and wattles- freeze- and cause frostbite. 'Ventilated' does not mean drafty. Drafts are very dangerous if in direct line of the chickens' roosts, but an air-tight coop is not good unless fully heated to above 40F degrees (I think fully heating a coop is bad- keep reading to see why). Better than a bunch of holes in the coop for ventilation, I prefer a tiny personal fan to keep air from being still and stratifying as they sleep. Many coops are built with windows- keeping those cracked can help. Also, soffit vents and small ankle-high vents (away from roosting or nesting areas) are other options.
Heat- So, what about heat? Is it okay to have a heater in your coop? Is it necessary? An owner will find many opinions about heat, so it's important to consider some factors that are relevant to your situation:

    • Does your climate get wickedly, extremely bitter cold?
    • Is it possible you will lose power for extended periods?
    • Is prolonged cold an unusual problem, or is it the common condition?
If you live somewhere that hangs in the upper 30's all Winter and has occasional spells of negative teens for a couple of weeks in an unpredictable fashion, such as our Kansas City area, having an auxiliary source of heat can keep your birds from suffering for that unusual stretch. If you live somewhere with 20F degree highs most of the Winter, you'll likely want your birds to be acclimated to bearing those temps consistently.
Power loss is a good reason to acclimate rather than just run heat to the coop all season. What if your birds are accustomed to 40F degrees in the coop and then you lose power for a couple of days (I can't help but think of our frequent ice storms)? Your birds will have only feathered out in down for the balmy 40F degree temperatures they've enjoyed all Winter, and now they can't endure the frigid outside on their own. Should you allow them to experience the cold, their systems will become adjusted right down to their built-in outerwear.
I do supply heat during cold stretches. I also do my best to better protect the birds with large combs so they don't have frostbite issues. My preference is a small, personal oil-filled radiant heater. There are flat-panels available with a 15x18" size, or something thereabouts. These won't use more wattage than a heat-bulb, but don't subject the birds to 24-hour illumination, which can damage their egg-production system. Who wants to sleep in bright light, anyway? I place the panel near the sleeping position of the birds with the most risk at night. These panels won't change the ambient temperature of the coop much, but can manage the comb damage for a couple of birds nearby.
I will also use an oil-filled radiator in my large coop to keep the temps around 25F degrees during those stretches of single digits or negative lows. It's enough to mitigate discomfort and not enough to cause them to molt those extra downy layers of feathers.
Heat of ANY kind can be terribly dangerous- follow all of the safety precautions recommended by the manufacturer and then some. There are coop fires every Winter, and safety is crucial! If using a clamp lamp, always put a screw in on each side to prevent the lamp from falling...even in your brooder.
Water, not ice- Heated platforms for the waterers or heated dog bowls work wonders for keeping their water drinkable. They must continue to have clean water available, fresh daily. An owner might find plans for various low-cost solutions to keeping a waterer thawed, such as heat-tape. In a coop which has no power, even this option isn't possible, so keeping extra waterers is a good solution. When letting the birds out (should they choose to go out in the weather) take along a new, fresh waterer with warm water and bring in the frozen one to thaw for the following morning.
Shelter from the wind and a place to scratch- Heavy plastic sheeting is your friend! Used to create a tunnel on the South side of a coop, or along the run, it can offer a great break from North winds. If it is used over the run, the protected area gives the chickens an area to scratch that isn't snow-covered and abrasive. If snow-packed earth is all they have, one can also put a thick layer of prairie hay on the snow to give them an area to mill about and sun themselves without freezing their feet. Picking through hay for seeds is a favorite pastime for chickens, to boot.