There was a lot of discussion on this relative to the first generation Birds - quite a few of them had it. Two good solutions have already been mentioned here, bar end weights and grip puppies. The other thing that helped a lot of us was a minor modification to the mount between the risers and the top triple clamp. In many of the first gen Birds (including my 2012), the tolerances stacked up such that the rubber vibration isolators in that joint were compressed too much and could not properly damp high frequency vibrations. There was a spacer tube, or pair of them in there, and using a washer to effectively lengthen the spacer by a little bit killed like 95% of the vibration. Mirrors cleared up and hands stopped going numb. There are threads here and on the other Thunderbird site about it.
You might also double-check to ensure you are keeping a loose grip on the bars. I would have sworn I was, but apparently the grunt of the Bird triggers some otherwise-dormant reflex and I will occasionally over-grip the bars, particularly the throttle, if I'm riding sporty. I've gotten in the habit of braking with middle, ring, and pinky fingers, keeping index and thumb for throttle. Riding that way, with the other fingers on the brake lever, seems to help remind me to keep my grip light.