To be electrocuted you must be part of a complete circuit. This means you must touch both a positive wire, and a negative or neutral wire (or “ground”). If a bird was touching the ground while touching the wire, the ground would act as a neutral wire and the current would flow through the bird, electrocuting it. If the bird sat on a wire and touched the metal of the pylon or another wire, it would also complete a circuit and get electrocuted. Because the bird’s only sits on one wire, it is safe.