What if the bird sitting on tansmission wire touches both terminal's wire? why

Because the electrical current is activated

There are no terminals on the wire where the bird is sitting. If the bird wants terminals,

he must move to one end of the wire, but he will find only one terminal there. The

other terminal is at the other end of the wire, conceivably hundreds of miles away.

No species of bird extant in the modern world can accomplish such a span.