Action: Remove earth wires to reduce incidental bird mortality
A before-and-after study and a literature review describe significant reductions in collision mortalities of cranes Grus spp. and grouse Lagopus spp. following the removal of earth wires.
Earth wires act to protect transmission wires against lightning, by dispersing excessive electricity. They are normally positioned either above or below the transmission wires, increasing the vertical height of wires and therefore the chances of collisions. In addition, earth wires tend to be thinner than transmission wires and so less visible, which, when they are positioned above transmission wires, increases the chances of collisions as birds climb to avoid the transmission wires.
Supporting evidence from individual studies
A controlled before-and-after trial between 1989 and 1995 in boreal and subalpine forests in southern Norway (Bevanger & Broseth 2001) found a 51% decrease in collision mortality of willow grouse Lagopus lagopus and rock ptarmigan L. mutus by a 2.5 km section of 22 kV power line from which the earth wire was removed (49 fatalities before removal, 24 afterwards). There was no corresponding decrease in two control sections (61 vs. 50 fatalities and 20 vs. 27 fatalities). The earth wire was located 1.5 m below the phase conductor lines.
A literature review (Jenkins et al. 2010) described a before-and-after trial (Brown et al. 1987) that found an 80% reduction in collision mortality of sandhill cranes Grus canadensis and whooping cranes G. americana following removal of the earth wire from a 3.2 km span of 116 kV line in Colorado, USA.
Brown, W. M., Drewien, R. C. & Bizeau, E. G. (1987) Mortality of cranes and waterfowl from powerline collisions in the San Luis Valley, Colorado. 128–136 Proceedings of the crane workshop, 1985 Platte River Whooping Crane Maintenance Trust, Grand Island, Nebraska.
- Bevanger K. & Brøseth H. (2001) Bird collisions with power lines – an experiment with ptarmigan (Lagopus spp.). Biological Conservation, 99, 341-346
- Jenkins A., Smallie J. & Diamond M. (2010) Avian Collisions with Power Lines: A Global Review of Causes and Mitigation with a South African Perspective. Bird Conservation International, 20, 263-278