They fluff up their feathers to trap in the heat and stay warm. While they don't need a heated house, they do need shelter that is dry and free from cold drafts. As the weather turns cold, make sure you are prepared by first inspecting the condition of your chicken housing. Are there any air leaks or holes? If so, be sure to make the necessary repairs to cover these areas to avoid heat loss and keep moisture out as well as small predators. The easiest and cheapest way to repair any holes is to screw a cut-to-measure piece of plywood over the hole.
Although you shouldn’t have any large holes in the coop to let cold air in, you also have to remember not to reduce the air flow too much, as this will lead to serious problems like ammonia build-up and illness among your chickens. To guard against this, you should ensure you have an adequate ventilation system in place.
Vents should be placed towards the roof of the coop, where the cold air doesn’t flow directly onto your birds. By venting out the warm, moisture-laden air and replacing it with cooler, drier air, you will be able to keep the humidity down and prevent mold from growing in your birds’ bedding.
Ideally, you’ll use a mesh vent with a hatch that you are able to open and close. This way, you will be able to properly vent the coop during the day and close it up in the evening when it’s colder, or during periods of particularly heavy rain.
During the wintertime, consider increasing the amount of bedding you place in your chicken coop. Straw is most commonly used, though some people use sand, as it’s a bit easier to clean up. Putting down some extra bedding will keep the chickens' feet insulated from the frozen ground during the day while they scratch around. Be sure to keep the straw or sand clean and dry and replace it often when it becomes soiled or wet from inclement weather (typically at least two times per week).
Winter’s extreme cold can certainly reduce egg production, but darkness is the main reason production begins to slow in late fall. Research on this topic shows that chickens lay best when they receive at least fifteen hours of sunlight per day. In northern parts of the U.S., natural daylight drops to under nine hours at the end of December. To optimize egg production, supplemental lighting in the coop is a must for the next three to four months until the days get longer. Try hanging a light bulb, controlled by a timer, in the nesting area to extend the 'daylight' hours to ~fifteen. With this method, egg production will be improved through the shorter days of winter!
Fresh water is necessary to keep your chickens healthy. Chickens need fresh, clean water all year long, but this becomes more challenging with daily freezing temperatures. If you have electricity in the barn, invest in a heated water bowl. But, be careful installing electrical items in the chicken house. If you don't have electricity, you will need to provide fresh water several times a day. During freezing temperatures, check the water source often because it will freeze quickly and the chickens may not be able to break it to get the water they need on a daily basis.
Staying warm requires more energy, so your chickens will need more poultry feed in the winter to keep their body heat up. Feeding the chickens later in the evening will also help keep them warmer throughout the night as their bodies digest the food.