You could put some hay down to bring warmth and make clean up a heck of a lot easier!
If I were you, I would not leave them with a dirt floor. Like Amiga said, a predator could easily dig under that and right into your coop!
It only takes one raccoon to wipe out your entire flock!
Ours have a dirt floor. We dug a trench around the edges and put down hardware cloth to deter burrowers. We cover the dirt with hay and change out the hay every few weeks.
Yes, I planed on putting wire around to deter digging predators. But instead of digging a trench and burying it, I was just going to lay it horizontal on the grounds surface, and attach it to the shelter that way.
i am thinking of a new place for the ducks and a dirt floor, but NOT plain dirt, bedding ect. will be placed down, just not a wood floor. Constructed more like a mini barn, as I have seen a lot of people mention their ducks are in their barn at night in empty stall....