Wintertime Care of Chickens
In terms of keeping your chickens warm, there is very little you need to do for your chickens in the colder months, as they naturally huddle together to share their body heat when they are in the coop at nighttime. Adding some extra bedding, such as straw is a good idea to make them more comfortable though.
You may also want to consider the use of a poultry wattle and comb protector for the winter, as in extremely cold weather a chicken's wattle and comb can be susceptible to frostbite - just a thin coating applied regularly will keep your chickens protected.
A mineral supplement is an especially good idea, especially if your chickens are molting, which they are inclined to do when it gets cold - look in the pet store for chicken spice, or poultry spice.
The use of citricidal (extract of grapefruit seed) has been validated by the FDA as an anti-viral to cure colds, and two drops in a pint of water will help protect your chickens, as will the addition of cider vinegar. Cider vinegar works as a general tonic and maintains a healthy appetite as well as keeping feathers in good condition - just 4 teaspoons to 2 pints of water is the ideal mix.
Not only will all these measures have a positive impact on your chickens in the wintertime, but will also keep them in tip-top condition when it comes to egg laying.