First, you need more roosting space for that many birds. Second, once you have the space, just keep moving the young birds that are roosting in the nesting box up to the roost each evening - trust me, it may take a week or more each night of doing it, but they will learn that it's nicer up there on the roost and stop sleeping in the nesting box. I was given this advice on this same forum and it did work. Some of them were really adamant about sleeping in the nesting box and took us ten nights of moving them, but ever since none of them have done it again. No more poopy!
