I'll echo what many have already said; "Bird shot is for birds!"
I have seen lots of data with pictures showing differences in buckshot vs birdshot on human tissue. One trauma nurse wrote an entire article on various gunshot wounds (I clicked through to it via this forum, does anyone remember what I'm talking about??) and he said conclusively, in his experience, that buckshot is preferred to get the job done. While bird shot wounds look ghastly, they are usually only that and suffer from severe under penetration, never hitting any vital structures. It will certainly make cheeseburger out of someone's face, but I don't believe that would be your first target anyway. Your instinct, hopefully, will be a hit to the center mass and 00 will take care of your problem rather quickly. I would bet that most bird shot victims end up in ER and not the morgue.
EDIT- Use what you have on deck. It's better than nothing. However, I really never understood the mixing of ammunition and not just straight loading the tube with 00. It is readily available and cheap. What's the deal?? Are people just trying to be different? Hmmm...
"Shut up, crime!"