 Following Crosby 's departure , Gene Clark briefly rejoined the band , but left just three weeks later , after again refusing to board an aircraft while on tour . There is some disagreement among biographers and band historians as to whether Clark actually participated in the recording sessions for The Notorious Byrd Brothers , but there is evidence to suggest that he sang backing vocals on the songs " Goin ' Back " and " Space Odyssey " . Michael Clarke also returned to the band briefly , towards the end of the album sessions , before being informed by McGuinn and Hillman that he was once again an ex @-@ member . 
