KEG: As Knight13 and screwdriver noted on this site many years ago, Bird's fatal mistake was 5...gxN. I do not agree that Bird would have been "on equal footing" with 5...QxN, but I agree that he would not have been lost with this much better move.
What is remarkable is that Lasker was able to exploit this error by Bird so effectively and obtain and retain a won game even though he did not win material until his 29th move! Bird fought hard and tried a number of ways to obtain counterplay, including an attempted Queen-side pawn storm, but Lasker gave him no chances.
Lasker could have shortened Bird's resistance with the clever 27. Bxa5! Both Lasker and Bird (and the Tournament Book) missed this pretty shot, and instead Lasker just played to win a pawn with 27. Qf3 and 28. NxB.
Bird, who had fought so hard after his mistake on move 5 to stay in the game finally blundered with 31...Ka7, which allowed Lasker to set up a crushing pin with 32. d5 and 33. Bd4--winning the exchange. The game did not last long after that.
Why did Bird play the ugly looking 5...gxf6? The Tournament Book thinks he was trying to establish a strong center, but the shattering of his position by Lasker with 6. Qh5+ was certainly foreseeable, and his pawn mass in the center was nowhere near adequate compensation for the demolition of his position that 5...gxf6 allowed.
Once again, Bird burdened himself with awful opening play. He might have gotten away with this against a lesser opponent, but here he was facing Lasker.