My own experience with 30-some years of grouse hunting in NY, VT, WI, ID, and WA leads me to agree with the "stats" quoted above.

I have hunted with guys who didn't follow much of any safety criteria, shooting "at" birds both seen and heard. Mostly they did about as well as the rest of us who shot a lot less often. (And nobody would hunt with them twice except their son or baby brother....).