Brian Jones played the recorder (it sounds like a flute) on this song. He was a founding member of the group and fancied himself their leader, which along with a debilitating drug habit, starting causing problems in the band around this time. He was booted from the group in June 1969, and found dead in his swimming pool less than a month later.
"Ruby Tuesday" is one of several early Rolling Stones songs that show his musical ingenuity. He was the group's lead guitarist, but could play just about any instrument. One of his most famous performances is on sitar for "Paint It Black