A bird in danger?

An electric current can only flow if there is an electric potential difference between two points, for instance the two feet of a bird.

But the electric potential of the high voltage line is the same on the whole line. So, sitting on this high voltage line means for the bird that there can't be any potential difference between its feet. No electric current can run through the birds' bodies.

If a person, however, climbs a power pole of a high voltage line, there will always be an electric current! For there will be an electric potential difference between the human body touching the voltage line and the ground! The voltage line has a very high potential and the ground has a very low potential. The potential difference is enormous! The electric current flows through the person's body; he or she has an electric shock.