I'd agree with everyone's answer of force fetch.
Watch the dog on the return and when you see her/him get ready to drop the bird try a verbal reinforcement like hold (if you used it during FF) or a quick heel.
Another one you can try is with a water retrieve. Start close to the water so the dog doesn't have time to drop the bird. Slowly increase distance.
And if you only have trouble with birds and not bumpers, then you probably need to use more birds.
Just my two cents.