When the Whippoorwill is a nonsense song that Mr. Clown sings when climbing to the top of the water tower in "Clown and Out". He sings it at the end with aliens after Wakko sends him to space. The only times we hear this are when the Clown climbs up to the water tower and when he's on Mars. The Martian children react much more enthusiastically to this song than Wakko.