A batch of billionaires, including David Koch, Alfred Taubman, Galen Weston, Stephen Schwarzman and Henry Kravis, attended a surprise 75th birthday for former Canadian Prime Minister Brian Mulroney on Saturday in Palm Beach.

Spies at private Club Colette said the birthday boy even jumped up to sing with musical act the Canadian Tenors in front of 150 guests, including Bob Colacello, Annette Tapert and Joe Allen, Alan Thicke, Terry Allen Kramer, Jackie Desmarais, Nancy Brinker and Mulroney’s wife Mila.

But David Foster — the Canadian singer and producer of hits for Barbra Streisand, Prince and Madonna — stole the show by crooning his tunes and “naming which of his three ex-wives got royalties from each of the songs,” a source said.

Politics proved to run in the Mulroney clan when one of the former PM’s grandchildren made a precocious toast to his “papa.”

“The 9-year-old mentioned that Brian went to Nelson Mandela’s funeral and how much good his grandfather did for mankind,” said an impressed spy.