Difference though is what your birds are accustomed too. Mine unfortunately, are stuck here... so must deal with the cold, my calls still swim daily but runners aren't they from a warm place? My scovies are tricky as the temps drop, my mallard derived are way more resilient to super cold, the calls again being the most hardy duck i have ever seen.Our ducks (three mallards in two coops attached with a screen between them – so they can see each other without fighting – and an open front door so they can get out to their heated water buckets) have been down to 0 F without any problems. But, it's the usual provisos: make sure they have plenty of food (they will be burning extra calories to keep their body temperatures up), shelter out of the wind, with plenty of dry straw to sleep on. The floor of the coop should be off the ground both for purposes of ventilation, and because the ground can be a pretty big heat sink, even through the straw.
Tonight, however, we're due to get down to -6F, and at that temperature, we break down and bring them into the basement for the evening.
I think you'll find Canadians on this list who will scoff at bringing them in for anything warmer than -15 F (-26 C). But our threshold is usually about 5F, maybe 10F if it's also going to be very windy.
Amiga mentioned something i had forgotten i too have a thermometer in my main duck barn, gives me an idea where we stand...