Hello, I am a first time proud owner of 1 orphanton, 1 Australop, and 1 silver lace Wyandotte chickens. They are all about a month old. My question / problem is that I live in the city San Francisco, and our backyard is all cement. My chickens love taking baths in their chick crumble. What can I use to substitute for their "baths"? What kind of dirt? Or is it ok to use playground sand?