I suppose you could hunt woodcock or grouse with 9s, but, I never have.

I met Don Zutz at a skeet shoot at my local club, when I was a 30 something, and bought a few of his books. He had written what I had spent my time discovering to that point, a 1 ounce load of 6s out of something that you would hunt in the upper Midwest was about perfect.
I shoot some 7 1/2s at grouse early in the season, depending on what I have and what is on sale. But, for birds, that is the minimum.

7 1/2 or 8s at clay targets.

Not sure I have 9s in the house.

Best,
Ted