Have you ever wondered why birds and squirrels don't get zapped by electricity when they sit on electrical wires? It's because they are not touching the ground, or in other words, they're not grounded.
It helps to understand the nature of electricity. Electricity is always trying to reach the ground and if an electrical conductor, such as a power line, provides the path, electricity will follow it.
When a bird lands on one wire, the bird's legs and the wire are equally charged, in other words, they are at the same voltage level or "potential.” Most birds do not get electrocuted because they are at the same potential as the lines they are sitting on and therefore do not create a current flow.
The same is true of squirrels when they land on the wire.
The voltage level or potential changes when a bird or squirrel touches a part of the structure going to ground. Suddenly, part of the animal will be charged and the other part won't. In that case, a bird or squirrel would get electrocuted, just as a person would.