Sand, about 5 to 6 inches deep. It wont wash away nor get muddy...think beach. It also dries quicker and deters parasites. It's easier to scoop poop and no more nasty mudpuddles that birds love to drink from.
I've heard the sand must be a particular type or grade. Dawg 53 - or anyone else - can you weigh in on this please? I've got a dirt run I realize needs to be changed out to something better, something that will keep out moisture after rain. Thank you!!