We live in Southwest VA where it does get cold in the winter but can also be really mild. The days lately, for example, have been in the mid 60's with nights falling into the 40's. We originally planned our chicken tractor to be able to be moved from the coop to various spots in the yard in the morning and back at night. But, as often happens, reality didn't match up with intentions, and moving it across the yard repeatedly is a HUGE pain! The tractor has a nice little nest box on it, and the gals will huddle in there together at night. At what temperatures though do I need to worry about making sure they can get inside the coop to stay warm? We have a Buff Orpington, a Dark Brahma, two Partridge Cochins, and two Ameracaunas.