Hardwood sawdust for mammals, alot of guys will use corn cob gritfor their birds because it is easier to get out of the down feathers. I don't like to tumble my birds, I feel it beats them up too much. What I like to do is after the degreasing bath, place the bird in Acetone ( safer than white gas) which will displace the water and any fine residue of remaining oil, from there I will rub borax into the feathers which will dry the feathers to about 80%, then I follow up with a bird drier to complete the drying process.