12 Gauge: Birdshot vs Buckshot vs Slug
If you want to see the difference between birdshot, buckshot and slugs, watch this video. As a testing medium, big chunks of meat are perhaps more representative than ballistic gel. However, cold meat, devoid of skin, is still not a particularly accurate test medium. But better than bare gel.
The birdshot only penetrated about 3" into the 4" chunk of meat. That's far short of the FBI's recommended 12" penetration depth. Think about it this way: in a shooting, can you guarantee that every shot you may ever take will be square on against a lightly built individual, with no barriers between you and him? A fat criminal could take multiple blasts of birdshot and have nothing get through his layers of fat. An extended arm could absorb the vast bulk of the projectiles and energy before they make it to the trunk, let alone the blood bearing, vital organs. A chair or other barrier could likewise render a load of birdshot ineffective.
Also, these shots look like they were taken at about 4-5 yards - acceptable for "indoor" comparison purposes. But if that distance stretches out some the bird will start to lose even this degree of effectiveness, while the buck and slug would remain far more effective. At really close range, I would use birdshot if I had no other choice, and would do so knowing that it does pack a severe wallop - but I would have to be very selective with my shots. If I had a choice I'd load buck or slugs.