Many of us keep chickens, ducks, geese, turkeys and game birds year-round. However, despite the hardiness of our feathered friends, most birds lose a lot of production because of a few missing tricks that flock owners can use in winter to help keep their flocks warm and thriving.
The three most important things to keeping flocks warm in winter are species, housing, and feed. I live in an area where we get snow in October, and lose it in May, and we get -40° temperatures! However, using this information I have kept my flocks laying and even brooding chicks for many months of the winter.

Species

Chickens

Many of us own more than one species of bird. However, all can be climatized well to cold temperatures.
Chickens are usually very hardy. If you live in a cold area, breeds that have small combs, a medium to large body weight, and even feathering are a must. Breeds like Orpingtons, Plymouth Rocks, Chanteclers, Wyandottes, Australorps, and mixed breeds are very hardy. Bantams, feathered-leg chickens, and large combed chickens are not very hardy.
Bantams: Bantams are usually very small and as a result do not cope in cold well. They produce less body heat, so tighter housing is necessary. Many bantams also enjoy flying, which can make them hard to coop up in winter.
Feathered Feet: Feathered feet can become frozen or snowy. Although it seems like feathered feet would be warmer, they usually accumulate snow and ice balls, making a chicken's toes pretty frosty. This also applies to loose feathered birds like Silkie chickens, who often become "feather iscicles".
Large Combs: Large combs are made to dissipate heat, not contain it. Many a beautiful comb have become stunted after a winter of frostbite. Smaller combs are less prone to frostbite.
Long Tails: Long tails also accumulate snow and ice, so if you are keeping ornamental birds like Pheonix or Sumatra chickens, keep in mind that snow may ruin the tail quality.

Ducks and Geese
Ducks and geese are very cute, happy animals. Even in the coldest of weather they are ready to play and make a mess. This can prove problematic for waterfowl owners.
However, ducks and geese don't roost (excluding Muscovies), making housing them tighter far easier. Many ducks and geese are more cold-hardy than chickens, because of their feathering and lack of combs. Medium to large duck breeds are suggested for cold climates. Ducks also have partially numb feet, so they rarely mind snow- but this means you should check for frostbite regularly. Because ducks always play in water, even at -30°, you should check for hypothermia regularly!
Muscovies: Check muscovies regularly for frostbite on their caruncles. Muscovies also need to roost, don't forget!
Bantams: Call ducks and other small ducks (3 pounds or less) are much more prone to frostbite and cold. Give these birds tighter housing and deep bedding.

Quail, Guineas, Turkeys, and Pheasants
Quail are very cold hardy, adaptive little birds. If given bedding such as straw, shavings or shredded newspaper, they will huddle down and enjoy the cold like little troopers. However, more exotic or "wild" breeds should be cared for with caution- you will need to reach in the coop area regularly, so be careful that they don't "flush" up and break their necks as quail are known to do.
Guinea fowl and turkeys are in similar boats- both are very cold-hardy, but must be watched for frostbite on their heads. Because of their bare heads, some may be prone to getting cold, so keep an eye on them.
Pheasants are very hardy and do well even in the most stone-age of structures. Provide them with roosts, food and water, and bedding and they'll do well.

Housing
All species will require some form of housing, even in summer. However, in winter, your requirements are going to be a lot different. If your temperatures get below -20° (-4°F), you'll want to keep your birds cooped up most of the time, so make sure you have lots of space (6-10 square feet per bird). You'll have to decide if you want to insulate, what bedding to use, and what the birds need to stay healthy. Also keep in mind that in winter, you are more likely to run into predator issues, as wild predators are far hungrier this time of year!

Ventilation
Your flock, no matter what species, must have ventilation. As humans, our first instinct is to close up every window or open space when the weather turns cold. However this is detrimental to any bird. Birds produce a lot of moisture, which in turn makes high humidity. Winter is very low humidity, for good reason- it allows us to survive in temperatures as low as -50°C. If winter had high humidity, the moisture in the air would become frozen and kill us, or make us sick. So keep your coop well ventilated. Ventilation on one side (try to avoid wind) is the best way to keep the coop ventilated but draft-free. Remember: airy, but no drafts or wind!
Insulation
To insulate or not to- this is a hard debate. When building a coop on a budget, most people opt out. However, insulating has its benefits. If you live in an area where it gets below -10° celsius, insulating can increase egg production, decrease frostbite, keep it warmer in winter and cooler in summer, and increase longevity. It can decrease food consumption as well.
There are many ways to insulate. I insulated my coop, and so far in -40° to -45° celsius weather I have had minimal frostbite, 8 thriving eight week old chicks, and 4 eggs per week (even with only 8 hours of daylight! I have 4 mature hens).
When building, most people make their coop double-walled. Include the roof too, because heat rises, so it will escape easiest there. In between walls, you can use fiberglass insulation (what I used- make sure you use a vapour barrier with this), shavings, straw (hay will go moldy), old blankets, newspaper, etc. If you want to save money, don't insulate the door or floor.
If you have an already built coop, you can do external insulation. This can be achieved by stacking straw or hay bales around the coop, covering it with a tarp, or both.
When building, if you live in a climate where the temperature regularly gets below -20°C (-4°F), include some (not all- force yourself to always provide chickens with ventilation) covers for ventilation. This can be achieved by making little "doors" for ventilation- I have done this, and my coop stays warm but surpisingly, still very cool in our hot summers.
Bedding
Bedding is important when keeping chickens or other fowl warm. For dry fowl (chickens, turkeys, guinea fowl, quail and pheasants) I regularly use shavings. They are easy to aerate, cheap to buy ($4 CAD for 6 cubic feet here), smell great, and are versatile. For wet fowl (ducks and geese), don't use shavings as they will get too soggy. Straw or wood pellets are better. For quail, even shredded newspaper or rabbit litter works. Game birds thrive in shavings or straw.
As much as sand is loved by many bird keepers, in cold climates sand will be your enemy. Sand can become wet and freeze, and it is hard to aerate, making it high maintenance in winter, especially when you might be moving food and water inside.
I use the deep-litter-method to keep my birds warm and also reduce bedding costs. With quail, though, I would never do this as they don't accumulate enough poop (unless overcrowded).
I do my DLM by fully strip-cleaning the coop twice a year- spring and fall. Every time I start with a new "rotation", I start with 3" of bedding. As I care and check on my birds I kick it up and aerate it. When I even faintly begin to smell anything (or even before- never a bad idea), I add another sufficient layer of bedding. Usually an inch or so. I mix it in, maybe add some more on top and keep repeating the process.
Space
When temperatures get below -20°C (-4°F) I always bring my birds inside their coop to reduce frostbite etc. If you plan to do the same (and you may very well have to in winter), you'll need to account for the extra space the birds will need. They will be using the coop as their temporary living area. 6-10 square feet of bird is sufficient (make it ten if you have a game bird like a pheasant, game bird chicken, Sumatra, or turkeys) until they get back outside. Because of this, I always maintain my own flock numbers at an appropriate level and cull or sell when needed.
Quail regularly only need 2-4 square feet per bird, and in winter this really doesn't need adjustment as they don't live in captivity the same way other birds do.
Walls
Although any coop in winter areas, or cold climates, requires a roof, many species of birds are content or warm enough with a three sided shelter.
Birds like chickens, turkeys, guinea fowl, Muscovies, and bantams should have a four sided enclosure.
Ducks and geese should ideally have a four sided shelter, but the right breeds will still thrive in a three-sided shelter.
Pheasants will survive in a three sided shelter, but they need freedom from the wind and sufficient roost protection.
Quail often thrive in structures that are three sided and out of the wind, but they need cover from the wind, bottom, and top.
Other Methods
Other ways of keeping your poultry warm are varied, and some like the heat lamp are very polarizing.
If you have windows on your coop, covering them with a blanket (still providing necessary light, of course) can provide additional insulation. If the sun is shockingly warm one day, then you can remove them.
Heat lamps are a good way to warm up small birds or injured birds. Young chicks and bantams may need them in extremely cold environments. Although many are worried about fire hazards with heat lamps, if the heat lamp is 18" above bedding and dust-free, there is minimal risk. Be aware that using a heat lamp often requires you to reclimatize birds.
However, do not heat up the air in your coop. Heat lamps are so successful because they heat up objects and not the air. Heaters that heat the air cause birds to become unused to cold, so if a power outage hits or your birds go outside they could easily become ill, frostbitten, or die.
If you don't provide a heat lamp, on very cold days your birds may huddle together or in a nest box to stay warm.

Feeding
This will be a long section so buckle down and take notes, folks!
We all know feeding and nutrition are essential to the quality, growth, and flavour of a flock's produce or individuals. The better your feed, the better the flock, most of us agree.
In summer, many flocks are allowed to get extra nutrients and protein from grass, bugs, seeds and other forage. Some people feed twice a day, and others all day. In summer, I keep my food outside, and it seems most people do the same.
Now it's winter time and below freezing. It's time to ditch those previous habits that are so important in summer!
There is no need to worry about obesity, too many "treats", etc. in winter.
Here is an example of what my chickens get fed:
In spring, summer, and fall they get 24/7 access to maintenance feed mixed with whole grains(17-18%) with free-choice grit and oyster shell. Their food and water is kept outside the coop, in the run, and they get full access during the day but also enjoy a variety of forage in my 400 sq ft run. They also get vegetable and fruit table scraps. I feed any growing chicks 18% grower, and game birds get a 26-30% ration.
In winter time, there is no forage for them, but they'll scrape through my hay in the run area for a bit when it's warm enough. I mix whole grains for winter, including a fair amout of corn because it generates good body heat. They still get table scraps, and get 24/7 access to food and water inside their coop. Chicks get grower.
So why do I feed them so much in winter? Well, in winter time, the more food they get, the more body heat they produce. If they are ever cooped up, I also never have to worry about if they've had sufficient feed. Incorporating scraps and corn helps produce more body heat than regular crumbles or other whole grains. Feeding them as many table scraps as you want also replaces their summer forage, without negatively affecting them.
Another reason I change feed habits from maintenance-grain mix to whole grains in winter is because they are producing less, and although they still require all basic vitamins, I can provide a changing variety of grains which can either help warm, fatten, or slim them without lacking nutrients. Whole grains are cheaper in my area than maintenance feed, so I also save during winter by feeding whole grains. However, I have to be careful that my birds aren't lacking certain nutrients, especially in winter-time when they can't forage.

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