The reason why bird dont' get electricuted is because they don't ground themselves.
The entire wire is setting at some pretty high voltage, but simply setting on 'a' wire will not cause harm because the electricity
prefers to take the more conductive route provided by the wire.
T------------------------T
You can be anywhere on that wire but electricity prefers to travel through the conductive wire and not through body. But suppose any part of you body touches elsewhere like another wire or ground, than you form a 2nd path for the electricity to take. The difference of voltage between the wire you are touching and whatever other surface you come in contact with creates another route for the electricity to go through. Although the vast amount of electricity will stilll goes through the wire and take the more conductive route, an significant amount of electricity will travel through your body to get to that point where you are touching. Even contacts between two wires of the same Voltage RMS will produce sparkes because it is a 3 phase power system meaning at any one time the voltage between the 3 wires is most likely different.