This is an easy one, right? Because both of the bird’s feet are on the wire no electricity flows through it. There is no circuit, its two feet are at the same electric potential, and electricity travels along the wire instead of through the bird, so the bird doesn’t get shocked.
This is probably what you were taught in physics lessons at school. But just as your maths teacher lied to you when he or she told you that you wouldn’t have a calculator with you all the time, your science teacher also didn’t tell you the whole truth.
The answer taught in school is a simplified one and only applies for a steady state systems, when the voltage doesn’t change. If you’ve ever touched a car and got a shock, you’ll know that in cases where the voltage does change, the answer is incorrect.
The science
When a car is parked there is no circuit as the car is insulated from the ground, yet current is still able to flow through you to the ground via your feet which gives you a shock. The car has been charged to a high voltage and when you touch it, it discharges through you to the ground. In this case no circuit is required for the charging currents to flow; it’s the change in voltage which occurs when you make contact with the car that causes the shock.
Power lines carry alternating currents or AC where the electric current regularly changes direction. Over time the voltage alternates between positive and negative indicating the direction of voltage flow. Remember, voltage is the difference in charge between two points, whilst current, measured in amperes, or amps, is the rate at which the charge is flowing. The number of cycles per second is measured in Hertz (Hz) and power lines in the UK have a frequency of 50 Hz.
Because the voltage in the wire is constantly changing, the voltage of a bird sitting on a wire is also changing. In other words, the bird is constantly being charged and discharged through its feet.
The other thing we need to think about is the capacitance of the bird. This is the ability of the bird to collect and store electric charge. Because birds are very small and shaped a bit like spheres, they can’t store much electric charge. In fact, when a bird sits on a 50 Hz power line it can only carry a current in the order of microamps (μA), or 0.00000001 amps.
So although the bird does have a current running through its body, it is too small to electrocute it or even give it an electric shock. For comparison, a current of about 10 milliamps (0.01 amp) is enough to produce a painful shock in humans, and electrocution is possible from about 50 milliamps.
We don’t know whether birds perched on power lines can feel the current going through their bodies. Perhaps they enjoy the buzzing sensation that they may be feeling and perhaps this explains why you don’t often see big birds, with a larger capacity, sitting on power lines.
The impact of power lines on birds
Although most birds are safe when perching on power lines, tens of millions of birds a year are killed from flying into power lines. Many of these birds are not electrocuted or shocked but simply die from the impact of the collision. Other birds, though, do die from electrocution and this happens for a couple of reason.
Transmission power lines carry electricity at a much higher voltage of anything between 275 kV and 400 kV in the UK, and up to 800 kV in some parts of the world, compared to distribution power lines. A high voltage power line can be strong enough to ionise the surrounding air and create a voltage gradient that increases the nearer you get. You can see this in action when aerial linemen approach a power line with a conducting rod and sparks fly before it makes contact.
If a bird gets near a high voltage cable it will feel a sensation from the generated electricity and should avoid landing on it. However, if a bird is flying at speed or is unable to manoeuvre away from the power line in time, the voltage gradient may be big enough to cause a current to flow through the bird, resulting in its electrocution, or a shock so big that it falls to the ground and dies that way.
The other problem occurs when two parts of a bird’s body touches two different wires of different voltages or when the bird touches a wire and a grounded part such as a pole or pylon at the same time creating a short circuit. The difference in voltage causes a current that flows through the bird that’s strong enough to kill it.
The birds most at risk of electrocution from power lines are fast-flying, large birds such as raptors, owls, pelicans, flamingos, storks, swans, and geese. Some wildfires have started due to birds hitting power lines and falling to the ground in flames. Conservation organisations across the world are working to identify and implement preventative measures to try and reduce the impact power lines have on birds and their surrounding environment.
credit link:https://www.birdspot.co.uk/bird-brain/why-dont-birds-get-electrocuted-sitting-on-power-lines
