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

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. 

Because the electrical current is activated