Via TFG, a video showing the effects of various shotgun ammo.
Again, birdshot is for birds. Not saying I’m volunteering to get shot with it (it still won’t be pleasant), but for self-defense it’s buckshot for general use and slugs for distance.
Some of those exotic rounds were interesting to watch.
Stuff From Hsoi 
Though birdshot would not be my first choice, I would not discount it in home defense. Catching a load of birdshot at ranges found inside the home (usually twelve feet or less) could cause considerable damage, depending on shot placement. The thought of a load of birdshot to the groin is not pleasant (and would definitely discourage a potential rapist), a shot to the throat could be lethal if just one BB hits the jugular or a carotid, and shots to the face would cause at least blindness, and possibly death.
Birdshot is better than nothing at all, and like I said I’m not volunteering to get shot by it. But that’s a lot of “if’s” and “could’s” in what you wrote.
If it comes time for us to have to shoot, then we need to shoot to stop. Birdshot could stop. Even buckshot or a slug isn’t guaranteed to stop. However, chances of getting that stop and getting it sooner rather than later is increased using buckshot or slugs. So if I have to use a shotgun, buckshot in general with slugs in the side-saddle if a longer shot needs to be taken. Birdshot only if I had no other choice.
Or of course, I’d rather use other things than a shotgun in the first place.
I will say tho… that one exotic ammo in the video of the buck and slug combo? That’s interesting.