A bird perches on a bare high power line and nothing happens to it. A man standing on ground touches the same line and gets a fatal shock. why?
You need some background in electricity before you can understand this answer so please look at the link at the beginning of the sentence first.
The current running on the power line is in an electrical circuit . A generator has two wires: one provides the energy and carries the high voltage, and the circuit closes going back to the ground where the other end of the generator is.
In order for the bird to have current going through it, it should have a second connection to close a circuit through its body. As it is sitting at one point it does not feel much, although there should be some ionisation of the air for very high voltage.For large birds it can happen that if a wing touches a second power line, the bird not only fries, but can set fire to a forest.
A person on the ground touching a power line closes a circuit to the ground, which is a universal point for the return of the current.
To get shock the electrical current should pass through a body. Due to Ohm's law this current is directly proportional to the potential difference across the two points. Taking into account electrical resistance of the bird's body, the potential difference between its legs is much smaller than a potential difference between the wire and the ground. Here is more explanations and illustrations: http://www.ibiblio.org/kuphaldt/electricCircuits/DC/DC_3.html