Rowly, the cylinders are light and very portable. I have had 3 and 4 ducks trying to use them at the same time! Every 15 feet would be fine and I suspect that they could be closer. I have by trial and error found that having the cylinders resting among natural plantings makes them more popular. The ducks do like to pluck the straw out of the cylinders so I do have to periodically go around and stuff some more in the outside. You may find that a few stakes will keep them in place if you have a windy situation.
You might be interested in hearing that my new Khaki Campbells are starting to lay already. They are supposed to be the most prolific layers of all fowl including chickens.
Jim
