Question

why birds dont get electric shock when the sit on power lines

Answer 1

Because they do not touch anything that will transmit electric current from them to earth.

Answer 2

$\huge\textbf\red{Elo!}$

When they are sitting on electrical lines, any part of their body is not connected with earth or second phase and because of non completion of circuit they don't get electrocuted. If any part of their body touch with second conductor or earth while one already connected to another phase they get burned badly like cooked bird.

✌️_✌️