Use whatever materials you see in nature and try to emulate that....bark, wood shavings, pine needles, woody weed trimmings, leaves, dried grass clippings(if you live in a hot and arid climate). Whatever encourages bug and worm life, keeps the barren soil covered, creates a depth of carbonaceous materials with which the nitrogen of the feces can bind and decompose properly. Keep adding to it as time goes along to keep that depth and proper balance of materials to promote good composting of all the materials.
It's the equivalent of giving your chickens a big compost pile to live on, with all the healthy bacteria and insect life that they need underfoot and in their habitat. Imagine the feral chickens of tropical climates and how rich is their diet when they forage under the canopy of a tropical forest. Think of how deep that forest floor litter really is and how rich in bug life, how clean the soils are so that they are loamy, spongy and able to absorb water instead of letting it stand on the top to grow harmful bacteria and attract pestilent mosquitoes. Their climate may be humid and damp,but their footing is dry due to all the leaf and twig litter on which they are walking.
If you live in a dry climate, use bedding that absorbs or has natural moisture, like grasses, hay, etc. If you live in a humid climate, add more dry materials that decompose more slowly, like bark and shavings.