Current temps have been in the 30-40s over the past few weeks, with the odd warmer/colder day here and there. However, tonight, temps are expected to drop to -5 with wind chill. I don't normally keep a heat source in my coop and my chickens have weathered the cold just fine. However, I'm concerned that this drastic change (currently 46 degrees, down to -5) may be a bit too much for them to handle. Should I add a heat source for the night, or the next few days? Other than a heat lamp I used for them as babies, I don't have any other real options. I'm concerned that could be a safety hazard, but the plunging temps could be just as dangerous? Advice?