This expression had been made popular by Peter Sellers when you look at the movie "The celebration". It was supposed to stimulate a parrot to have interaction with Peter and consume the seeds that Peter was "bombarding" the bird with. It don't work, but the phrase life on thru the fans of "The celebration" which use "Birdy num num" for hardly any other reason rather than tell one another associated with motion picture for them to start crackin up !!!