I'm no expert but I believe that it's about creating a carbon environment (wood shavings, leaf litter, etc) which will neutralize the nitrogen in the droppings to create a neutral environment and help surpress the ammonia which can be so harmful to the lungs of our chickens.
Another vote for pine shavings. My chicken guru tells me that the mold in hay is bad for the chickens.
Does anyone know of a good kind of wood chipper that would make shavings? I'm tired of paying $5 a bale
at the Tractor Supply.