Because wearing insulated shoes should birds not jerk when parked on the power cord? Not so.
Current is the flow of electrons through the wire. It always follows the easiest route, ie always find the least resistance path to run through.
Electric cables are usually made of copper. Copper has the lowest resistivity and is a very good conductor.
Meanwhile, the bird's body has a higher resistivity and is much less conductive than copper. Therefore, the current will bypass the bird's body but transmit through the power cable. As a result, the bird does not jerk.
In addition, the current flows from the highest voltage to the lowest voltage. Power cables often have different voltages. If the bird's legs are on the same cable, they have the same voltage. Therefore, the bird does not get jerky.
However, if the bird puts on two different cables (with different voltages) the current will flow through the bird's body from a high voltage cable to a lower voltage cable causing the bird to be electrocuted.