Unless you're planning on being attacked by birds, don't use birdshot as a defensive round.
Even at across-your-kitchen distance, birdshot is going to leave a messy but shallow wound. I would expect that in many cases (other than point-blank) you could probably walk away from it.
I mean it's better than nothing, but since you have the choice of what to load in your defensive gun, choose buckshot or slugs.
Some interesting shotgun ballistics:
http://www.shotgunworld.com/bbs/viewtopic.php?t=109958