Good answer Karl! And you were wise not to include the information below in your response. I figure the forum is the right place to insert the “yeah, but” items…
If the wire isn’t insulated, then there will be a very small current flow through the bird’s legs. Since the wires under discussion are not perfect conductors, there is some small amount of resistance in the wire between the bird’s feet. When current flows through a resistance, a voltage will be generated across the resistance. This “voltage drop” across the half-centimeter of wire between the bird’s legs is very small, but non-zero. So a minute amount of current will flow through the bird.
In other words, it’s not quite correct to say that electricity will take the path of least resistance. It is more correct to say that MOST of the electricity will take that path. If the wire is a million times less resistant, than the amount of current that goes through the wire will be a million times greater. But that still leaves a millionth of it that ends up going through the bird.
All of which causes the bird to say, “Big deal.”
Remember that the current in the wires in question is alternating current. It cycles the electric charge 60 times per second. When the wire is negative, some electrons will flow from the wire into the bird’s body, which will then quickly reach electrical equilibrium and stop flowing. When the wire then changes charge and becomes positive, it will attract the electrons in the bird’s body, which will therefore flow back out and into the wire. Then the wire turns negative again and electrons flow back into the bird again. This periodic movement of electrons into and out of the bird constitutes a current flow.
As with the voltage drop, the actual amount of current is small … at least for a bird. However, if YOU were to hang by that wire, you would probably feel a buzz. This is because you have a lot more mass than the bird (or so I assume) and can therefore take in a lot more electrons before you reach electrical equilibrium. The more massive you are, the greater number of electrons that will flow each time the wire’s charge switches (60 times per second). If a few of your best friends hang from your feet, you might end up with a large enough current flow to injure you.
Now, just to throw a twist into the mix, let’s say the wire is insulated. Therefore no electrons can actually pass between the wire and the bird’s body. No more current, right? Wrong. Although the effect is not as great as if the wire were bare, there will still be a small current, due to the capasitive effect. Again, let’s say the wire is negative. No electrons can jump from the wire to the bird’s foot, BUT the electrons that are already IN the bird’s foot will be repelled by the negative charge in the wire. Those repelled electrons will migrate up the bird’s legs and into the bird’s body. When the wire switches to positive charge, electrons in the bird’s body will be ATTRACTED to the wire, and will flow down through the legs to the feet.
In this case, there is a current flowing, but it is caused by the electrons that are already in the bird. They flow back and forth through the legs, being alternately repelled and attracted to the wire. Even though there is no direct connection. Again, this effect is small for a bird and would be greater for a larger body (due to the greater number of electrons available to move about). But the effect is unlikely to shock you, even if you have a few friends hanging onto you.
Another source of current flow is from the magnetic field around the wire. Again, this is dependant on the electricity being alternating current. As the wire switches its charge between positive and negative, a magnetic field builds and collapses around the wire. One property of a moving magnetic field is that it will induce an electrical current in a conductor placed nearby. Note that for this effect to be present, the conductor DOES NOT EVEN HAVE TO BE TOUCHING THE WIRE! The magnetic field growing and shrinking around the wire will induce small eddy currents throughout the bird’s body, even if the bird is hovering a short distance away.
Since magnetic fields can penitrate most common forms of insulation, even an insulated wire can induce these currents. Grab hold of a lamp cord and you too will have these small currents induced in your body. But these currents are also VERY small and can’t be felt.
Of these effects, only #2 will generate an appreciable amount of current, and even that would not be enough to bother a bird.