St Michael's Mount: bird on a wire

There were power lines across our view of St Michael's Mount and the birds came to perch. I was interested that although most chose to perch on the top wire that is the Earth wire and therefore carries no current, one perched on a lower wire. The theory is that since each foot is at the same potential, no current can flow through the bird because a potential difference is needed for current to flow. This is true but I seldom see birds perched on the lower wires - almost always on the Earth wire at the top.