Our ducks were inside most of their young life, but have since been put outside. It's starting to really get cold, we're supposed to dip below freezing for the first time tonight. Their house seems to stay warm (they even fog up the windows), but I am concerned about them. I've seen people say they do and don't need heat, but I am not sure. So what do you think? How cold tolerant are adult ducks? At what point should we add a heat source to the house?