Birds don’t get electrocuted on bare (and very much live) power lines because they stand at only one node on a power line. A bird is effectively just standing at one point of a circuit, thus, there is no potential difference across its body. And when there is no potential difference (or voltage) across a bird’s body, no current can flow through it. No voltage, no electrocution.
Then how does one get electrocuted?
Electrocution happens when there is an electrical potential difference between two points of one’s body. This electrical potential difference (also known as voltage) is what drives current through one’s body. The greater the voltage, the greater the current, the deadlier.
If birds don’t get electrocuted by touching only one power line, why do humans get electrocuted when touching only one live wire at home?
Humans do get electrocuted touching a live wire when they are also in direct contact with the ground. In power distribution system, the ground (the literal earth ground) is part of the system. Apart from the Hot and Neutral lines where current normally flows, there is a third line called the Ground line. This ground line is connected to the power source’s neutral terminal down to the actual ground. It serves as a safety measure when there is an electrical ground fault.
In normal conditions, current flows from the Hot line to the Neutral line. When there is a ground fault, the current flows from the Hot line to the Ground line. A person touching the Hot line (bare) and in contact with the ground (either standing on the ground barefoot or touching the concrete wall) is an example of a ground fault. In this situation, the current flows from the power source to the Hot line to the person’s body to the ground line back to the power source (and vice versa because it’s AC). In other words, the person’s body becomes a load just as a washing machine does under normal power system operation.
Briefly, humans touching a bare live wire (the Hot line) do get electrocuted only when any bare part of their body is also touching the ground or anything connected to the ground that can conduct electricity. When a person has no direct contact with the neutral line or the ground, he should be safe. That is essentially what the bird situation is — they are standing on one live power line, but not touching any other power line or anything connected to the ground. Otherwise, the poor bird dies.
An argument on the bird situation
It was mentioned that birds don’t get electrocuted because they are standing at only one point of a circuit, hence the absence of potential difference. Actually, that is not entirely true. In this bird situation, the bird is not literally at only one point on a circuit. The bird has two feet and each one is on a different point. In an ideal conductor with no resistivity, this is not an issue — the feet’s contact on two points can be considered one point (or node) because there is absolutely no resistance between them.
However, in the real world, power lines are never ideal. Aluminum, the material used in power lines and one of the best conductors, exhibits a certain amount of resistivity. Therefore, there is resistance between any two points on a power line. With a conductor’s resistance and current flowing through it, one can be sure that there is some voltage drop across. This is Ohm’s law in action. When another element (say, the bird) gets paralleled across those two points of the power line, there would be a voltage drop on the parallel network (composed of the wire and the bird). Although most of the current would flow through the near-zero-resistance wire, there would still be a certain amount of current that would flow through the high-resistance bird. (Yes, the statement that “current tends to flow through the path of least resistance” is false and misleading.)
All those things considered, how do birds survive? That is because the amount of current that flows from one bird’s foot to another is too small (and I mean really, really small) to cause harm or even be felt. The current through the bird is too small because the voltage across the bird is also too small. The voltage across it is too small because the resistance of the power line is also too small.
In other words, even though the bird’s two feet are a couple of centimeters apart, the cable’s resistance between those two contacts would be too negligible to acquire a significant voltage drop to shock the parallel-connected bird.
[A]