I think you have slightly misquoted a line from the song 'Somewhere over the rainbow' which was performed by Judy Garland in 'The Wizard of Oz'. I believe it should be 'if pretty little blue birds fly'. To answer your question which I assume was asked tongue in cheek: Rainbows are formed so high up in the atmosphere that it is probably only possible to fly over one in the space shuttle. If you have actually seen bluebirds flying over one I would say that this is probably an illusion caused by your position on the earth's surface, the bluebird's position in the sky and the relative position of the rainbow. If however you are conducting research into how it might be possible for you to fly over the rainbow I wish you every success in your endeavours.