AceShowbiz - Former U.S. President Bill Clinton was treated to a musical surprise on his 64th birthday - Sir Paul McCartney called him up to serenade him. The Democrat was delighted after picking up the phone to hear The Beatles legend on the line after reaching the age on Thursday, August 19.

And he was stunned when McCartney began an impromptu rendition of the Beatles' 1967 love song, "When I'm Sixty-Four". source tells Britain's Daily Mirror, "Sir Paul knows Bill is a huge fan of the Fab Four and instantly agreed to ring him. Well, it's not every day you can get on the line to a former President."

"But Bill was even more over the moon when he heard him - and couldn't believe it when he started singing When I'm Sixty-Four. Bill's been a fan his whole life. He loves Macca's solo stuff, too. So they chatted for ages."

It's not the first American leader McCartney has performed for - he staged a concert for U.S. President Barack Obama at the White House in Washington, D.C. in June.