During the winter, on days that are below 32 degrees farenheit (a few degrees below won’t hurt) you should not let your chickens out of the coop.  Even though they can still go back in the coop, the cold air will make the coop colder as well.  Also, chickens, like humans, can get frostbite, (except they can get on their comb!) when they are out in below-freezing temperatures for a long period of time.  The advice I an giving for keeping your chickens warm during the winter:  Like I said before, keep the chickens in the coop on days below freezing.  This way, no cold can get in, and with all the chickens generating body heat, if you have enough chickens, the coop should be fairly warm.  The last two ideas are having a heat lamp on the ceiling to keep the coop extra warm, and make sure you give the chickens corn every day.  The corn helps them generate more body heat.  Hope these tips help in keeping your chickens warm this winter!