In order for electricity to harm something it has to pass THROUGH that something. Electricity needs a closed “circuit” to flow. And it prefers “the path of least resistance” to do that flow.
So electricity is flowing along a copper or other highly conductive (electricity-friendly) wire. The path of least resistance for the electricity to flow is the wire itself, because air doesn’t conduct electricity nearly as well.
Neither do a bird’s legs. Let’s look at a diagram.
..............BIRD
...........BIRDBIRDbird
........BIRDBIRD
....BIRDBIRDBIRD
BIRDBIRDBIRD
...BIRD.BIRD
....L....L
....E....E
....G....G
WIREWIREWIREWIREWIREWIREWIREWIREWIREWIREWIREWIRE
Electricity loves the WIRE more than it loves the LEGs and BIRD parts. So electricity stays with the wire. The bird is safe.
Now let’s do a man working on a transmission tower that accidentally touches two different, and wrong, wires. The arrows represent which way the electricity is flowing.
WIREWIREWIREWIREWIREWIREWIREWIREWIRE---->
..........\......
...........\O....
............|\...
............|.\..
<----WIREWIREWIREWIREWIREWIREWIREWIREWIRE
The man’s body has CREATED A NEW PATH for electricity to flow… and if that new path helps the electricity complete the circuit, dude is fried. The electric current is dangerous because the man is big enough to bridge the gap and his body is friendly enough to electricity to allow it to pass through him. It’s more conductive than the air, so it’s enough to bridge the gap and give the electricity a path.
See Also
- - Titan Implosion
- - Single Phase Power
- - ELI5: Food Poisoning
- - ELI5: Human Progress
- - ELI5: Insects and sunlight