I think because it was a big fast dog it had a better chance of catching roos, I've seen two German shepherds out bush on different occasions and they were dying of starvation. What surprised me was I thought greys needed a fair bit of care. The grey was in heavily timbered bushland with no farms for miles but probably creeks around. There was a Jack Russell that lived in a state forest here, everyone used to see him and he lived for years, feral as. Being small he probably lived on rodents and lizards.