People have been using down feathers from ducks and geese to make warm jackets and blankets for themselves for centuries. That same down keeps the ducks nice and warm. I am with Weeg, and would not add an electric heater. On really really cold nights though, you can fill a large bottle (like a milk jug) with hot tap water. As the water cools, and eventually freezes, it will give off heat. You can also buy a cheap, remote thermometer. I think I paid $10 at Walmart. You then place the probe in the duck house and the receiver in your house. Any time you are worried you can check the temperature by the ducks. I was surprised, using mine, to find the house many degrees warmer than the outside air.