Why can birds sit safely on power lines?

The reason that birds can sit unharmed on power lines is that no current flows through them because, by sitting on one line only, they are not completing the electrical circuit. Rather like water flowing down hill, electricity needs to flow down an electrical hill, otherwise known as a potential difference. So, if you connect the plus side of a battery to the minus side, current flows because there are lots of positive charges at the plus pole and very few at the negative, so the current flows to try to equal things out. Returning to the bird sitting on a power line, no current flows through the bird because it is at the same potential as the wire it is sitting on, so no current flows. But, if it were to straddle a positive and a negative line, it would certainly be curtains! Ditto if it put one leg on the ground and another leg on the live wire. Under these circumstances, there is a big potential difference between the wire and the the ground, so electricity would flow through the bird in an attempt to balance things out. ZAP!