The birdshot for self defense thing has been done to death. Bird shot is for little birds. Anything less than #1 buckshot is either gambling with your life (non-ideal ammo for a justified shooting) or asking to spend lots of time in a non-voluntary government resort (your life wasn't really in danger).
Lots of people have this idea that they should use birdshot or rock salt in a shotgun for warning/less lethal purposes. This is an extension of the "force continuum" concept. The problem legally is that that once you are past the threshold for potentially lethal force, using wounding/maiming force shows both a desire to hurt rather than stop an opponent (may not be true, but that's what the lawyers will argue), and a lack of a real threat (why the hell didn't you shoot to STOP if he was really such a threat?). If you truly *have* to shoot, shoot to stop the threat, not piss it off or scare it. Otherwise keep your finger off the trigger.
As for why birdshot is undesireable for technical reasons as opposed to legal ones, take a couple of gallon jugs of water out to an outdoor range, and set them up on a table or other torso-high stand, laying them four deep. In order to have sufficient penetration to reach vital organs and rapidly STOP a determined attacker, you need to penetrate completely through at least three, preferably four jugs. Anything less than that isn't going to penetrate to the vitals of a 300lb drunk fatso/rapist/burglar/thug/drug dealer/etc except under optimal conditions. You have to plan on shooting through thick clothing, maybe a half-battered down bedroom door, his arm, and through into the vitals. The FBI figures that as about 12-16" worth of ballistic gel, which we approximate as 18-24" of water.
Figure a jug is 6". I'm betting that most, if not all of your shot will only penetrate the first jug, and fail to reach the second, unless you are within arms reach of the target. I have done this with #6 shot, and failed to penetrate to the second jug. Take a relatively wimpy gun like a .380 or a light target load for a .38 special. Hell, even a .22lr. Any of them will punch right through at least three jugs, probably all four. I've done this test with a .38 using both standard pressure and +p, FMJ and hollowpoint. To have enough penetration for stopping a perpetrator, you're going to have enough penetration to exit your home and punch through at least one of your neighbor's walls across the street. There has been no satisfactory solution to this problem since firearms were invented except for making sure that your shots don't miss.