I can give you my opinion of runners - they are great, and my flock of ten need to be where it's 40F or more at night in order to have optimum health. I live in New England, and I used to live in Ohio. They go out during the day if it's in the 20's or more, they need to be able to come in where it's a little warmer, and for overnight. This is just my flock, I cannot tell you about all ducks.
You will find many people who say that ducks are fine at minus some large number of degrees, out in the wind and snow and ice.
I hesitate when people say, "ducks are fine," because that doesn't tell you what breed, how many, the temperature of the coop, and other details.
I would recommend that you have a backup plan in case your ducks don't turn out quite as cold hardy as others or the book say they are.
