It's likely that they might see the power lines' magnetic "signature" before everything else, as if the lines are highlighted or marked by a visual filter. Larg

It's likely that they might see the power lines' magnetic "signature" before everything else, as if the lines are highlighted or marked by a visual filter. Large congregations of birds on power lines preparing for a flight, however, is not a clear indicator for that. Perhaps power lines are just more convenient than searching for a tree large enough for the whole flock.