Electric power transmission lines are extremely dangerous for living beings. However, nothings happen to birds sitting on transmission lines, Since they are touching only one phase and potential difference across their bodies is zero.
But, what it happens if someone touches the transmission lines?
This video illustrates what can happen to live beings if they touch two phases at once.
Warning: This video displays a monkey that was accidentally electrocuted while jumping on transmission lines. The video is shared for knowledge purposes only.