I kind of always thought that whistling in a song was a bit out in left field. Would it really work or not?

Well I was wrong and of course Jim proved that in his excellent presentation of "Adios Amigo".

Other songs with whistling that absolutely worked would be the 1957 Marty Robbins hit "The Story of my Life". And I have to mention "Sittin' On The Dock of the Bay" by Otis Redding. And of course Ray Conniff provided a lot of arranged whistling that to this day is enjoyable. And so many more and they all worked well. Who knew?