For any living being to get electrocuted, a current of sufficient voltage has to pass through its body, i.e. there should be completion of the electric circuit. Birds perching on live electric transmission lines are not killed because they are in contact with only on wire and no current passes through them. But if the bird sitting on a live-wire simultaneously comes in contact with the ground or touches another live-wire, the electric circuit would be complete and the bird will be killed by the current flowing through it. Similarly a person hanging on to a single live-wire will not be electrocuted unless he touches the ground or the earth wire.
