Birds don’t get electrocuted when they sit on electrical wires because they are not good conductors of electricity. They sit on only one wire and do not complete the ‘circuit’, that is touching both the positive wire and a negative or neutral wire. They would get electrocuted if they were touching the ground. The ground would act as a neutral wire and the current would flow through the bird. If the bird sat on the wire and touched the metal of the pylon or another wire, it would also complete a circuit and get electrocuted.