I like what the Box O Truth site has to say about using birdshot for defense against humans:
Birdshot as a Defense Load
I have had a lot of questions, summed up as follows: How effective is birdshot (#4, #6, #8, etc.) as a defense load?
We have done tests with various birdshot loads. Birdshot penetrated through two pieces of drywall (representing one wall) and was stopped in the paper on the front of the second wall. The problem with birdshot is that it does not penetrate enough to be effective as a defense round. Birdshot is designed to bring down little birds.
A policeman told of seeing a guy shot at close range with a load of 12 gauge birdshot, and was not even knocked down. He was still walking around when the EMTs got there. It was an ugly, shallow wound, but did not STOP the guy. And that is what we want... to STOP the bad guy from whatever he is doing. To do this, you must have a load that will reach the vitals of the bad guy. Birdshot will not do this.
In fact, tests have shown that even #4 Buckshot lacks the necessary penetration to reach the vital organs. Only 0 Buck, 00 Buck, and 000 Buck penetrate enough to reach the vital organs.
Unless you expect to be attacked by little birds, do not use birdshot. Use 00 Buck. It will do the job.
However, it remains a free country (for the time being). If you want to use birdshot to defend against humans, please feel free to do so.