I would never, ever, ever, ever load a round into a gun that has little to no stopping power when there are other rounds for that particular gun that have good stopping power.
When I first took my shotgun hiking in Grizzly country, I read some forums and took some bad advice. They said the first round should be birdshot, followed by buckshot and follow that by slugs, then alternate buck and slugs. I didn't see anyone say that was a bad idea, so that's what I did. Then I was talking to an Alaska State Trooper friend of mine (he's been featured on the show a few times) who asked my why my first round was birdshot. I said so that I can try to scare the bear instead of killing it, and if he keeps coming, then I can unload the buck and slugs.
He nodded, then asked me this, "How many rounds do you think you'll get off on a charging bear before he gets to you?" I said that I honestly don't know. He said, if the gun is slung on your shoulder, you'd be lucky to get one off, and if that one didn't stop the bear (bears don't always respond to pain or noise, especially if they're hungry, defending food, or with cubs), he'd be on top of you clawing your face off. He then said, if you give him a face full of birdshot, there's a great chance that even if it does scare him off, he'll lose sight in maybe one, probably both eyes, and perhaps even his sense of smell. You've still killed the bear, but this way it'll take a month or two for him to starve to death.
He said when you're in bear country, don't carry anything that won't have a great chance of stopping the bear. Later that year, I actually ran into a bear, and was about 10 feet from it before I noticed it. Luckily he ran off, but if he hadn't there wouldn't have been any way I would have gotten more than 1 shot off. That sold me. And by the way, when you're out in the woods, bears are hard to spot. Every encounter I've had (and I've had a few) has been within 20 yards. If my gun was slung on my back and the bear decided to charge at the moment I saw him, I wouldn't be able to get more than a shot off.
Sorry, not going to take my chances with bird shot against a bear.
And for those interested, my Trooper friend recommends Brenneke Black Magic 3" Magnums for bears. That's what the troopers use, and that's what I used.