I have 10 Indian Runners. I have 6 females and 4 that are straight runs. They're too icky to keep in the brooder box much longer so I'm working on their house. What size nesting boxes will they need? My duck book is totally useless, saying "some ducks nest on the ground, some should have nests at least 20 inches off the ground." Also, what is the ratio of nest boxes to ducks? Thanks!