The tambourine is not considered to be a musical instrument by any authority on music. Every musician knows that the tambourine is used by singers to occupy their hands in an attempt to trick the audience into believing that actual musical ability is present.
Anyone on stage brandishing a tambourine while not playing another musical instrument (ex. drums) or singing could be subject to severe mocking and, on rare occasions, physical harm (as happened on July 15th 1975, when Greg "Sassy Pants" Johnson of the Johnson Brauts attempted a tambourine solo cover of the James Taylor song, "You've Got a Friend," but quickly realized that the tambourine could not produce the correct tones (as it's not an actual musical instrument). Mr. Johnson was booed off of the stage where he was then severely beaten and violated by the crowd.