Grasp lower jaw with a leather gloved hand.. palm down in the mouth thumb under bottom of jaw while saying hold. It will take awhile for her not to try and spit out the hand but will eventually accept your hand. Make sure she holds your hand in your mouth calmly before progressing to birds. I also introduce some bird scent in the glove after she has started to accept the glove Then when a bird is retrieved hold the bird in her mouth while saying hold - keep on a short check cord so you can reel her in quickly if she starts to mouth the bird. You could slip in the bumper/wing between the glove and the birdget a round hairbrush and have her fetch that
The glove works great to teach them how to hold softly. I do it while watching TV with him in my lap as soon as he comes home from the kennel. Only takes a couple of minutes per sessionI have worked with Chesapeake Bay Retrievers (notorious for hard mouth) for 25 years. I wouldn't worry about. David is right about your dog still being a puppy. Just don't let her fetch fresh birds for a while. She's going to chew on whatever she can until those adult teeth are in place. Then the gloved hand trick Citori16 told you about is all I have ever needed.
Oh heck I've been trying hard to avoid giving internet dog training advice.Again you are not dealing with hard mouth in this situation!
Gatzby..can you chime in hear please??