Revisionist history for the most part.
Myth # 1: Bird had a greater peak
Bird's best statistical season would be Duncan's 5th best statistical season. Duncan's season-by-season PER blows Bird's out of the water, and then some. And stats don't even factor in defense. Duncan was the defensive backbone of the team with the best record in all of pro sports for a full decade. Bird's Celtics played in an era when defense was an afterthought, and Bird was average at best on the defensive end.
Myth # 2: Bird did more for his team.
Bird played on a team stacked with HOFs. Their only consistent compe ion was the Lakers. Teams like Philly and Houston had a year or two of prominence, but that was it. Duncan played in a way tougher conference where there were almost always 4 or 5 50+ win teams with a chance to make it to the Finals. Duncan's 2003 playoff campaign saw him leading his team in almost every statistical category with no current All-stars by his side.
Myth # 3: Bird picked it up for the playoffs more than Duncan
Bird's playoff PER is a full 2 points lower than his regular season PER. Duncan's playoff PER is 1 point higher than his regular season number. Only 2 of Bird's playoff runs had him post a PER above 22; in contrast Duncan had a PER above 22 in 9 of his 11 playoff runs. Bird's best playoff PER doesn't make Duncan's top 5 PER playoff runs.
So now that we've got that out of the way, let's point our Bird's real advantages:
1. Bird was a better leader, more motivated, had more fire
2. Bird was a better passer and floor general
3. Bird was more clutch in the waning minutes of big games
Those are all valid advantages, but they don't make a big enough dent in Duncan's overall body of work. Duncan was a top 3 offensive player and a top 3 defensive player for 8 straight years, Bird wasn't.
Bird's MVP awards don't mean much when you consider the way MVP award voting happens. The best player on the best team normally wins it, not the best overall player. Steve Nash has 2 MVPs to Shaq's 1, for Pete's sake.
Bird has a clear edge in intangibles, which is why this is even close. If Duncan had played for Boston on a stacked team, and Bird had played for the Pacers or Spurs without multiple HOFers, Duncan would be top 3 of all time and Bird would be struggling to crack the top 10.
Duncan is the best forward in NBA history. Bird is #2, and that too because Dr. J didn't have good enough teams around him all the time (and played in the ABA for 5 years)