jwp,
Contrary to what a lot of hunters believe, it's easier to kill bigger birds with the .410 than smaller birds. This is because they're bigger targets, which results in more hits from the relatively sparse pattern. This is also why the 28-gauge is even more effective than the .410 on larger birds: The shot charge is only a little bigger than the .410's, but the 28 patterns more evenly--with less shot-stringing, so more of the shot arrive at the same time.
At the other end, the 28 works better than the .410 on smaller birds, where more pattern density is desirable. It's also why 28-gauge averages on skeet (pretty small targets) are significantly higher than .410 averages, despite the relatively short range.
