I have discovered that rather than letting grass grow wild and free on the floor of your outside aviary, remove all the grass and the topsoil to a depth of about 100mm (5 inches). Lay plastic down on the ground and puncture it full of holes. (this is to allow the water to drain) Cover the plastic with approximately 100mm of 19mm clean stone. This allows you hassle free walking when you are in the aviary and makes washing the aviary out an absolute pleasure.