Why do birds sitting on electric wires not get shocked?

You will not get a shock unless you complete the circuit to ground. This is why power lines can be worked on while live, from a helicopter:

Helicopter Power Line Maintenance

A simple googling would've provided you an answer. We won't get a shock if we fly or when we aren't grounded... Because, Current flows only in closed circuits (Maybe in Plasma "as an open"). A bird sitting in the wire doesn't form a complete circuit in order for the current to flow. In other words, Birds have their feet in the same wire (It also has high resistance, Now, that's another problem).

I've seen many days, birds (like "crows" in our country) get shocks. Sometimes birds too touch the Phase and neutral (or Earthing) wires thereby creating a largest potential difference, get toasted & fall down. Not all birds are clever.

Imagine: Take a circuit with battery. Connect one terminal of the battery to a resistance and a galvanometer. Leave the other terminal open. If it shows deflection, then you'd solve your question and you'd definitely get a Nobel...

Have a look over this overview of the topic...

Because birds stand on a same electric wires, they are at an electric potential. The reason why people can be shocked, is because a person's body is a conductor, and when we touch the wire, there is a high electric potential on electric wire. But when birds stand on wires, there are always on the same electric potential, so they won't get shocked.