It is 18'F here now. Yes it's cold, but winter is cold. All my research told me that ducks are hardy creatures that can tolerate cold **as long as they have a nice shelter with deep straw to get to. And all my ducks survived last winter. I had a pair of Pekins that actually preferred the snow. As a general rule, I do not provide any supplemental heat sources either.
Now your ducks look younger than mine and that is certainly something for you to consider as you decide how to best manage your flock.