I wouldn't set lice and mites as a cause of death. Free ranging birds will dust bath when they feel they need to, no reason to provide it for them. Chickens have an amazing defensive mechanism at hiding sickness. Often times when they do show signs of something amidst, it's to late to do anything about it. People can add really anything they want here to include myself, but its all throwing darts in the dark. Only way you will know is to have a necropsy performed by a qualified vet for avian disease or your local agricultural extension. How is your water situation when they are alone all day? Is it provided and you know they have it throughout the period when you are working? Heat stress in chickens is insanely fatal and fast.
