I would consider roofing, and slope the roofing to a gutter that empties into a rain barrel, providing some extra water for the ducks' swimming. I have been thinking of roofing part of the day pen, also, just to keep the bedding dry during wet times.
I currently use a very large tarp - more during the winter to keep from having to shovel snow out of the pen.
I currently use a very large tarp - more during the winter to keep from having to shovel snow out of the pen.