My take on it is you cant predict how a dog will react when catching live quarry. i was taken under a relatives wing when i was young regarding training/retrieving etc he used to compete in gundog trials, i used to go through from the dummy stage through to cold game etc for retrieving. My dogs would retrieve live apart from one who crushed everything from the first to the last age 11 i dont think i could have done anything to prevent it or cure it it was just the dogs makeup he retrieved well but it was just fit for ferret food.