HUP is a book written by James Spencer, and would be a good start for you to read. One of the reasons is he covers retrieving in greater depth than most Flushing traning books.

Hup is also the command given to flushers, similar to the whoa with Pointers. The correct response to the command should be an instant sit and stay, regardless of the situation. Most commonly applied upon the flush, so that your dog can mark the bird when it falls.

Since you mentioned that your Springer does it with bones as well, I would think if you exerted your dominance a bit more and firmly put your dog in it's place .. this type of behavior would be less. I have found with guys I have helped that stressing obedience and control normally corrects this behavior, especially if the dog has been trained to retrieve properly.

I feel I should add that not all dogs are going to retrieve like the dogs on ESPN during field trials. Not moving a muscle and expecting your dog to lay the game at your feet sounds really good, but the reality for 75% of the guys out there is that a lot of work and calling and coaxing and praising are usually part of the package to get a dog to deliver to hand. Since your dog does retrieve, just has problems with the release, work on the obedience as I mentioned, and perhaps give some reward either praise or special treat, when a bird is delivered to hand. Only after the growling stops though .. do not reinforce that behavior.

Good luck, Springer's make great hunter's.