Birds are generally not electrocuted when sitting on power lines due to a few key reasons:
Lack of potential difference: Power lines typically carry high voltage electricity, but birds sitting on a single power line experience the same electrical potential. Electrocution occurs when there is a difference in electrical potential (voltage) between two points, allowing a current to flow. Since the bird is at the same potential as the power line it's perched on, there is no potential difference for current to pass through the bird's body.
Insulation: Power lines are designed with insulation materials, such as ceramic or plastic coatings, to prevent the electricity from flowing into the surrounding environment. This insulation acts as a barrier between the electrical conductor (the wire) and the bird, reducing the risk of electric shock.
Path of least resistance: If a bird were to touch two power lines at the same time, then it would be in contact with different potentials and could potentially create a path for current to flow through its body. However, birds generally avoid touching multiple lines simultaneously, thus avoiding the formation of a complete circuit.
It's important to note that while birds can safely sit on power lines, they can still face other dangers, such as accidental contact with other conductive objects or coming into contact with different voltage levels or electrical equipment outside of power lines. Additionally, larger birds with longer wingspans or birds that perch on objects that are closer to multiple power lines may face a higher risk of bridging the potential difference and experiencing an electric shock.