Birds can sit on power lines without getting electrocuted because they usually touch only one wire at a time. Simply being in contact with a live wire does not automatically mean electricity will flow through their bodies. For a dangerous electric current to pass through the bird, there needs to be a significant voltage difference between two points of contact.
Electricity follows a path between points of different electrical potential. When a bird lands on a single wire, both of its feet are essentially at nearly the same electrical potential. Since there is very little voltage difference across its body, there is no significant current flowing through it.
The bird’s body does not provide an alternative route for the electricity to travel. The power line is made of a highly conductive material, while the bird’s body is much less conductive. So, when both feet are on the same wire, electricity continues along the wire rather than taking a significant detour through the bird.