This is my first Iowa winter with chickens. I have 2 hens, 6 -18 week old pullets and 6 -18 week old guineas in a tall 12 x 10 coop with an attached 11 x 10 covered run. I live on 3 acres in the middle of farm land. All the fields are harvested, and I still have some greenish grass. Today it is 25 degrees with a wind chill to 19. No precipitation. When do you NOT let your poultry free range? What are some parameters?