People try to balance appearance, price, ease of use, and sanitary conditions.
Common choices include newspaper, pine shavings, ground corncob, ground walnut shell, kitty litter, sand, waxed paper, and plastic sheeting. The thing we must consider as top priority is the safety and well being of the animals under our care. It is natural for birds to pick, dig, and explore with their beaks. This presents a problem because discarded food and droppings are no more desirable in a bird's home than in your own. One thing that helps keep a bird away from debris in the cage bottom is a grate.