Birdshot penetrates about 4 inches in calibrated ballistics gel at 5 yards. As "nice" as that is, I like my defensive ammunition to penetrate more than 22short. Actually, I like my ammo to penetrate better than the average air rifle. CNS hits stop a badguy, birdshot WILL NOT penetrate that deep. Tearing up paper, treebark, waterballons, etc are not good indications of stopping power.
You would never hunt a buck with birdshot. So why would you expect it to stop a much more dangerous animal?
Cheney's hunting buddy after getting blasted in the face can tell you just how effective