Why, when birds sit on transmission lines or current wires doesn't get shock?

 Its true that if birds touch the single one line (phase or neutral) they don't get electrical shock... if birds   touch 2 lines then the circuit is closed and they get electrical shock..

 so if a human touch single one line(phase) then he doesn't get shock if he is in the air (not touching or     standing on the ground).

 if he is standing on the ground then touching the line (phase) he will