Dear Dulcimer, It's mentioned in "These Happy Golden Years." Laura sang the chorus "Whistle and I'll come to you my lad...", after which 'Manly asked her, "Would you?" "Of course not! It's just a song."' (From Chapt. 20; "Nellie Olsen")

As for the Burns, I found the lyrics in "The Essential Burns", edited by Robert Creeley (1989).

Sincerely, Mary Tickel