I usually put hay in there but they eat their beds when I do that. It takes a few days but I was wondering what others use!
Hay gets mouldy easily, and contains seeds and leaves which the chickens (and mice) will eat. That's why folks use straw (the stem of grain i.e. oats) and or wood shavings. Note "wood shavings" and not "sawdust" as sawdust packs tight and holds water where shavings are and stay loose.