A bird does not receive a shock when it lands on an electric wire because it lands on only one wire.  Electricity takes the path of least resistance—it flows through some materials much more easily than others.

It is simply easier for the electricity to continue along the metal wire than it is for it to detour through the bird.

If the bird landed on two wires with different voltages, however, the electricity would flow through the bird from the wire with the higher voltage to the wire with the lower voltage, and the bird would be electrocuted.–Dick Rogers