I'm the "bad" duck mommy. Mine are not quite a year old. We've had lows in the single digits and my ducks don't go in at night either. They have places that they can get out of the wind and snow, but they seem to like it.
I am not saying that i advocate this for others...do what you want with your own... I'm just saying that mine are out and not freezing.
They'll go in if they get cold...