why do birds,sitting on electric wires, not get shocked?

The birds do not get shocked because they do not complete an electrical circuit when they perch on the lines.There is no voltage difference in a single wire, electricity flows from areas of high voltage to areas of low voltage,moreover electricity flows through a path of least resistance and birds are not good conductors of electricity so the electric current bypass the birds and continue flowing in a single line. But if birds touch the ground while sitting on the wire or touch another wire with a different potential then their bodies will become a path of electricity to reach the ground or to a different wire then as a result birds can get electrocuted and may die.