we had below zero for several weeks off and on and low teens--I had a hard time keeping the coop between 25 and 30 degrees with 2 heat lamps running---but..... I have a large coop also(too big)
my chickens and ducks get plenty of corn, layer pellets and table scraps--this coop is also shared...
how are your winters? where do your temperatures drop to? I have to run 2 heat lamps when it gets really cold out just to keep it 30 degrees in the coop.--d.