Trailing Nelson by one, Tom Kites tee shot on the 167-yard 17th hit an airborne bird and fell into a pond. Kite eventually recorded a double bogey and fell from contention.
Nelson went on to birdie the 18th to finish the tournament at 15-under-par, three shots clear of Bruce Fleisher (66). Kite shot 67 to finish tied with overnight leader Mike Hill at 11-under. The 62-year-old Hill followed his Saturday 66 with a final-round even-par 72.
Nelson had only two bogeys on the Nashawtuc Country Club in Concord, Mass., en route to earning the $210,000 first-place prize.
Kite was the only player to battle Nelson on Sunday. He was alone in second place when he hit his tee shot at the 17th. The ball struck a purple martin, killing it and knocking the ball into the water. Kite took a drop on the bank of the pond, then chipped the ball onto the green and two-putted for a 5.
This is Nelsons third victory of the season, but his first since February. The Tour will now head to Peabody, Mass., for the 22nd U.S. Senior Open.
Full-field scores from the FleetBoston Classic