I'll toss in my two cents here - - -
When it comes to "ducks in winter" there are some variables that we don't always include in our answers.
Breed
Number of ducks
Their infrastructure - do they have constant access to a 40F barn? Are they in an unheated shed? How deep is the bedding?
A few of my Runners did not read the book that said they were fine even at 20F, and they started shivering, stopped laying, and were on the verge of miserable. Took a couple to the vet, who proclaimed them not so good at winter.
Moved the flock into the walkout basement that stays above 40F, and within 4 days, everyone was laying again, they were relaxed and content.
I asked someone who said their ducks were fine at 15F or so, and discovered that their shelter never got below 40F, and they could go into it any time - which they did.