Birds avoid electrocution when sitting on power lines because they lack grounding. However, if the birds touch the pole, which conducts electricity to the ground, at the same time they are on the line, the electricity could potentially kill them.
Electricity is the movement of electrons. In order to encourage electrons to move, there must be a potential difference. Most power companies use the ground as the return path for electricity. The electrons travel through the line to the appliance and complete the circuit through the ground.
When a human touches a live power line, he is often standing on the ground, completing the circuit and causing physical harm to his body. However, if someone is standing on insulating material, then he is able to touch the line without any physical harm.
Power company employees use insulating materials to their benefit when working on power lines. They insulate their clothing, tools and bucket trucks, so they do not conduct electricity. This protection allows them to touch the wires and make repairs without becoming a part of the circuit.
Electrical workers sometimes hang from helicopters when working on power lines, which allows them to work safely because of the lack of grounding for both the worker and the helicopter.