Which characteristic allows birds to safely perch on power lines?

The characteristic that allows birds to safely perch on power lines is the absence of a current path to the ground. When a bird sits on a power line, it is not completing an electrical circuit because both of its feet are on the same wire. Electricity travels along the path of least resistance and will only flow through a circuit when there is a difference in voltage that allows it to reach the ground, which does not happen in this scenario. Consequently, because the bird is not creating a path for the electric current to flow to the ground, it remains unharmed while perched on the line.

In contrast, being small or having insulation against electricity does not directly account for the safety of birds on power lines. Small size does not change the way electrical currents behave, nor does insulation play a prominent role in this instance. Additionally, low metabolism is irrelevant to the electrical interaction with power lines. The key factor is the arrangement of the bird's feet on the same wire, preventing an electrical path from being completed, which ensures the bird's safety.