I believe you can teach a dog just about anything.. An older dog would need more time IMO as a young one is more eager to learn. But if your dog listens to you well he will learn with paitence on your part. Here is a pic of Rusty retreiving a ruffed grouse, it doesn't appear to be a normal way to hold a bird but..
He was never forced to retrieve and with his natural way of doing it is fine with me.