I've never heard good things about crushed walnut shells but with that said, I've never used them and cant say from experience that they're bad. I've tried seeds and other stuff. What I've come up with is that there is no PERFECT substrate. It's probably impossible to mimic natural substrate conditions in indoor cages. But it seems that people do have success with mostly 3 substrates:
- Some use seeds, usually millet.
- Some use newspaper/paper towels/or no substrate
- Some use sand/dirt/soil
I've tried most of these and a few others. All have their benefits and drawbacks. Some considerations with substrate is cage type and size. If you have a large cage it might be easier to maintain a more natural substrate. But medium to smaller cages may need materials that are more absorbent, easy to clean, and will not impact digestion or health. Some examples of cage types influencing substrate choices might be that deep trough or Rubbermaid cages hold seeds well and front opening (glass door) cages work well with newspaper type.
My favorites are dirt/soil for larger cages and larger animals and no substrate for smaller cages/smaller animals. Fortunately for me out here in Arizona we have gravely, sandy, desert soil that's perfect and easy to replace when cleaning.
