Owned and bred by Marylou Whitney of Whitney family racing fame, Birdstone had won the Champagne Stakes, a leading race for two-year-olds, at Belmont Park in 2003, but had not been a factor in the first two Triple Crown races (finishing eighth in the Kentucky Derby and skipping the Preakness Stakes) and was a 36-1 longshot when he upset the overwhelmingly favored Smarty Jones, taking the lead in the final furlong (201 m).
His sire Grindstone won the Kentucky Derby in 1996, and was a son of Unbridled, who had done the same in 1990. Unbridled was in turn sired by Fappiano, a son of Mr. Prospector, the tail-male ancestor of the vast majority of winners of Triple Crown races in recent years (22 out of the 24 races 1998-2005).
Birdstone's victory represented the first in the Belmont for trainer Nick Zito, whose horses had finished second in that classic five times. According to the New York Times, the next major target for Birdstone was the Travers Stakes, which he won despite being the fourth choice at 5-1. He was Zito's first Travers champion.