Anytime you let them free range you risk something happening. I have lost one in four years. My four dogs have been taught to leave the chickens alone. Accidents happen when you have animals, so I work with them all so that when something does happen I don't have to worry about anything. Yes, you can teach an adult dog.
My chickens have a big area they hang in. I do not let them out in my 2 acre pasture unless I am out there with the dogs. If the chickens see a hawk they will usually run to one of the dogs.