I wouldn't let him ever just carry around the dummy. He needs to know that the reason he gets the bird in his mouth is to bring it to you. Also when you have him on the check cord I would praise him only when he gets to you and you can get a hand on him. This way the only way he gets praise is when he is all the way to you. I also would praise him a little when he is close to you before you take the bird. Once you have the bird I would give another quick praise then move on.

I think this lets the dog know that the action that gets him the praise is when he brings you a bird.