Electricity flows by the movement of electrons through conductors. As copper is a good conductor of electricity, the copper in electrical wires allows electricity to flow easily along their surface.
Birds don’t get electrocuted when they sit on an electrical wire because they are not good conductors of electricity. Their body does not offer electrons an easier medium than the copper wires they are sitting on for electricity to flow through. Hence, the electricity bypasses the birds and keeps flowing along the wire. But if the bird while remaining seated on the wire comes into contact with the ground or another electric wire with a different voltage, it may get electrocuted. This is because a difference in electric potential (voltage) causes the movement of electrons. Electricity flows from areas of high voltage to those of low voltage.
The bird’s body would become a path for the electricity to reach either the ground (no voltage) or a place with a different voltage (another wire with a different voltage, for example).
Picture Credit : Google