This will be my first winter with my chickens and I am concerned about keeping them warm. I did install a flat 2x4 for their roost, sealed any gaps for drafts and have a large overhead opening for ventilation. My concern is I only have four chickens and their coop is very large so I can't expect their body heat to help warm it. My breeds are a Rose comb Leghorn, Sumatra, Polish, and a Silkie. Does anyone have any tips or suggestions without using heat lamps? The average temperature were I am located in the winter is 15 degrees, but occasionally drops bellow zero on some nights. Any suggestions would be so appreciated.