Did you or someone else do the Force Fetch?.Its hard to give advice without seeing first hand just how severe this is.Theres a few things you can try.Frozen birds,more controled fetching with use of a checkcord,theres also harnesses with studs on them,you strap the birds in the harness during your fetch drills to reduce the 'chomp'.

If someone else did the FF,I'd contact them and let them know your having a problem and get their thoughts.

How old is the dog and have lots of birds been shot for it?Sometimes its just a matter of time and birds under control with the checkcord.Some dogs do the head shake because they may have been winged or spurred by a cripple they retrieved once.Take some time and think why you think the dog is doing this.It could be a matter of just more controled retrieves,it could be just needing experience having more birds retrieved,it could be the dog has a rough mouth,it could be the dog has a really hard mouth.I'd have to know more before I gave more direct advice.Good luck,take care,Jackson..