Question: The American Thoroughbred race horse Birdstone won which leg of the Triple Crown in 2004?

Context paragraphs:
Landaluce: Landaluce (April 11, 1980 – November 28, 1982) was a champion American Thoroughbred race horse.  She was out of the first crop sired by Seattle Slew, 1977’s Horse of the Year and the 10th winner of America's Triple Crown.  Her dam was Strip Poker, by Bold Bidder, sired by Bold Ruler.

Flawlessly: Flawlessly (1988–2002) was an American Thoroughbred race horse bred by Harbor View Farm.  She was a daughter of 1978 Triple Crown of Thoroughbred Racing champion Affirmed and La Confidence by Nijinsky II.

I'll Have Another: I'll Have Another (foaled April 1, 2009) is a North American Thoroughbred race horse, bred in Kentucky, owned by Canadian businessman J. Paul Reddam and trained by Doug O'Neill.  In May 2012, ridden by Mario Gutierrez, he won the first two legs of the Triple Crown by taking the Kentucky Derby with a time of 2:01.83.  and the Preakness Stakes in 1:55.94.  On the day before the Belmont Stakes, he was scratched due to tendonitis, ending his chances of winning the Triple Crown, and retired from racing.

Alydar: Alydar (March 23, 1975 – November 15, 1990) was a chestnut colt and an American Thoroughbred race horse who was most famous for finishing a close second to Affirmed in all three races of the Triple Crown.  With each successive race, Alydar narrowed Affirmed's margin of victory; Affirmed won by 1.5 lengths in the Kentucky Derby, by a neck in the Preakness and by a head in the Belmont Stakes.  Alydar has been described as the best horse in the history of Thoroughbred racing never to have won a championship.  Alydar's fame continued when he got older when he died under suspicious circumstances.

Belmont Stakes: The Belmont Stakes is an American Grade I stakes Thoroughbred horse race held every June at Belmont Park in Elmont, New York.  It is a 1.5 mi horse race, open to three-year-old Thoroughbreds.  Colts and geldings carry a weight of 126 lb ; fillies carry 121 lb .  The race, nicknamed The Test of the Champion, and The Run for the Carnations, is the third and final leg of the Triple Crown and is held five weeks after the Kentucky Derby and three weeks after the Preakness Stakes.  The 1973 Belmont Stakes and Triple Crown winner Secretariat holds the mile and a half stakes record (which is also a track and world record on dirt) of 2:24.

Sunriver: Sunriver (foaled March 17, 2003 in Kentucky, died 2009) is an American Thoroughbred race horse who was a contender for the Triple Crown in 2006.

Summer Bird: Summer Bird (April 7, 2006 – December 23, 2013) was a champion American Thoroughbred racehorse, son of 2004 Belmont Stakes winner Birdstone.  He was bred by retired cardiologist Kalarikkal Jayaraman and his wife, retired pathologist Vilasini Jayaraman, at their Tiffany Farm near Ocala, Florida.  On June 3, 2010, Summer Bird was retired due to complications of a previous injury.

Three Chimneys Farm: Three Chimneys Farm is an American Thoroughbred race horse breeding farm in Midway, Kentucky established in 1972 by Mr. & Mrs. Robert N. Clay.  Widely known as one of the world's preeminent horse farms, Three Chimneys has been home to a number of famous horses including U.S. Triple Crown champion Seattle Slew, U.S. Filly Triple Crown champion Chris Evert, as well as Silver Charm, Chief's Crown, Genuine Risk, Point Given, Slew o' Gold, Capote, Smarty Jones, and Big Brown.

Seattle Slew: Seattle Slew (February 15, 1974 – May 7, 2002) was an American Thoroughbred race horse who won the Triple Crown in 1977—the tenth of twelve horses to accomplish the feat.  He is the only horse to have won the Triple Crown while having been undefeated in any race previous.  In the Blood-Horse magazine List of the Top 100 U.S. Racehorses of the 20th Century Seattle Slew was ranked ninth.

Henny Hughes: Henny Hughes is a thoroughbred race horse.  A foal of 2003, he was a contender for the Triple Crown in 2006.  He was taken off the Triple Crown trail in March 2006, then recorded three sprint victories in stakes races before failing in the Breeders' Cup Sprint, finishing 14th and last.  He was retired to stud at the end of the 2006 racing season.  His best-known offspring is the champion mare Beholder.

Supporting paragraphs: Summer Bird Belmont Stakes
Answer: the third and final leg