I've seen people using cat litter, but it's expensive and has all kinds of crap in it. But. It's made out of some kind of clay that absorbs moisture, so maybe search the forums for cat litter and see what people say about it.
If you use cat litter, you need the natural clay type that has no fragrance since the fragrance can harm bird lungs. I use this under my quail cages (mesh floor with poop tray) and it seems to help lower the odor and is absorbent - just not as absorbent as "modern" kitty litters. Here is what I have purchased at Wal-mart: