Why do birds not electrocuted ?

The birds perched on a wire stretched high-voltage electricity, why do birds not shock, so about the question of a child one day ...

An electric current I flowing through the conductive wire voltage, and it is evident that the bird legs are attached directly to the wire, but due to the bird's legs standing on the voltage that has the same polarity (Va = Vb) then the bird's body is not exposed to electric shock.

If only electrical wire touched the ground and the bird still perched on the wire, there will be a voltage difference between the legs of birds, so birds will be exposed to electric shock immediately.