How do birds sit safely on electric power lines?

This is possible because a bird only touches one line. If the bird were to touch another line or pole the electricity would travel through the bird, either to the ground or another wire. To do any work, even to harm a person or other living thing, electricity has to flow FROM something TO something. So it could flow FROM a wire TO another wire, or FROM a wire TOO the ground, or FROM a cloud THROUGH a person TO the ground. When a bird sits on a wire there is great electrical POTENTIAL, but it doesn't go anywhere so the bird is perfectly safe since the electrical potential has no path through the bird. Both above answers are correct, though, i have more to add. The electicity carries through to... usually nothing! Like the above answer, there is that potential though. If there were another wire which the bird was also touching, there would be problems and if it were to touch another bird, result is the same. Another interesting fact, I might add: If someone were to grab a power line with both hands at the EXACT same time, while feet were not on the ground and NOT, i repeat NOT touch anything else or put anthing on the power line, while already holding it, and then let go at EXACTLY the same time, without touching the ground first... That person would survive, no injuries! I might add, while most birds perch on electrical wires perfectly safely, they do present some dangers. In particular, large birds such as eagles have wingspans wide enough that they occasionally brush two parallel wires with their wingtips, which completes the electrical circuit and allows electricity to run through the bird's body, killing them. This is a serious problem in certain desert areas where the birds use the electrical poles for nesting, due to limited other options.