Question: who did the whistling in the muppet movie?
Answer: musician Andrew Bird, Andrew Bird

or actually a Muppet, which was why he always felt a strong connection towards them. This emotion eventually leads him to sing the Academy Award-winning song, "Man or Muppet", where the human version of Walter is portrayed by actor Jim Parsons. The scene is pivotal to the character's arc as it resolves Walter's internal issues with himself. The character's solo act of whistling in the telethon's finale was performed by musician Andrew Bird.  At the conclusion of the film, Walter deduces that he is a Muppet, adopts whistling as his sole talent and joins the group as their newest member.