When does the
current flow? If there are voltage or
potential difference, then the current starts to flow
from high voltage to low voltage. But when a small
bird sits on the power line, both feet are on the
same voltage line!

Therefore, there are no potential difference
between these two feet.
Let's say the power line is very low, almost
touching the ground, and a chicken is trying to cross
it. If one leg is on the ground and the other one is
on the power line, then there are potential
difference between these two legs.

Therefore, there is a flow of charge and
eventually the chicken will be barbequed like the
diagram above.
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