Duster cannot resist the call of music. He doesn't drag his double bass outside without scoping the concert, so there is no fanfare of a large instrument rolling over the stones when Duster approaches Fitz and stands beside the spirits.

He ends up sitting down - it's good to rest his legs after all, and he was feeling a bit tired. He remains there until Fitz finishes his song, after which he provides quiet applause that the spirits mimic.