All I know is it's probubly the breed, My grandparents-in-law live just next door and they raise Hamburgs (I, on the other hand, had orps and sex links). A raccoon ate all of my chickens and theirs are still all out running around happily. Theirs free-range most of their feed and they have had them for many many years. So I guess it could be genetics being stronger making a better chicken, but more likely I think it's just the breed.