I did find a birdshot gelatin test on the web. However, it was only of a light 1 1/8 oz field load of #4 shot. Still, the average penetration of the #4 shot at a range of 9 ft was 6.5 inches. And the wound is quite massive in size:
This sure looks like a lethal hit to me.
Now in contrast, I found a photo of a fellow who survived a hunting accident of being shot in the side with birdshot. I have no info about the range involved, but just by judging the size of this pattern, he must have been some distance from the shooter. Certainly you can contrast this pattern size here to the pattern size at 9 ft in the gelatin photo.
Still, though, I have got to think that this fellow must have been in terrible pain from this shot. The number of pellets here would also indicate to me that this was a rather small shot size. Remember that not all of the pattern would have hit him.