You want a product which will make it easy to see the constistency, color, volume, shape and number of the droppings. An easy to examine bedding will also enable you to see what food your bird is eating and what is winding up on the bottom of the cage. You will need to change bedding materials daily so that fresh droppings aren't confused with older ones. Once ingested, corn cobs can remain in the body for years before causing illness or death. Ingested corn cobs absorb moisture and swell, which can cause impaction, bleeding and death.
Baby birds can also develop bacteria and yeast infections from the ingested material.
When wet or in humid climates, cobs can grow Aspergillus molds which cause a respiratory disease in birds. This disease is difficult to treat and can be fatal. The cobs will also absorb moisture from your bird's droppings, promoting growth of organisms and hiding loose poops. It's a poor material for examining droppings. Walnut Shells - Walnut shells, when eaten, can inflame and irritate organs, causing internal damage, bleeding and death.