Sia and Kurstin originally wrote this for the movie Pitch Perfect 2
. She recalled to Rolling Stone
: "They rejected it and took another song of ours called 'Flashlight
.' That become a big thing through the movie, but I could not believe they rejected 'Bird Set Free' because to me it was such a big, anthemic, fun, sing-along-in-the-car song. I felt seller's remorse. I had grief. I feel connected to that song."