Why can birds sit on power lines without being shocked?

The short answer is they are not grounded. You can hang from a line also and not be harmed, but don't try it as if you do it wrong you can be severely hurt or killed. So what does "they are not grounded" mean? Electric current flows from high voltage to low voltage. The power line is high voltage. The earth, or ground, is low voltage. Anything in contact with the earth is also at the same low voltage. That is why there are insulators between the lines and the poles. The higher voltage the lines, the bigger the insulators. You can see this for yourself by looking at a high voltage tower versus a typical wooden power poll. When the bird lands on the line, it is only in contact with the line. There is no path to the lower voltage ground for the electrical current to flow thorugh. No current flow means no harm to the bird. People get into to trouble with power lines because they are almost always in contact with the pole or the ground when they, or something they are holding, touch the line, which means the current flows through them to the lower voltage. Note that the lower voltage does not have to be zero volts. It just has to be lower than the high voltage for the current to flow. It is not the voltage that kills you. It is the combination of voltage and current. No current or low current, no harm. When you scoot your feet on the carpet on a dry day and touch the door knob, a spark jumps and you feel a small snap. Your skin is charged up to 10's of thousands of volts, but there is very little current flow, so no harm is done. Hope this helps.