Why do birds not get electrocuted when they sit on the third rail?

In order for someone to be electrocuted by a live power line or 3rd rail on the train they must also be in contact with ground, or close enough for the power to jump to ground on it's own.

That is also why they can sit on a power wire, the insulators at the poles keep the power from reaching ground.

High voltage transmission lines can be repaired by a helicopter crew. There is a strict procedure to follow though. The helicopter hovers close to the wire and a ground between the helicopter and the wire is established by making contact with the wire through a high voltage probe. Once that occurs, the operator hooks a clamp to the wire for a permanent connection. At this point, the operator can touch the wire and repair it without getting shocked. When done, the reverse procedure is followed exactly.