Action: Mark power lines to reduce incidental bird mortality
- A total of eight studies and two literature reviews from across the world found that marking power lines led to significant reductions in collision rates or dangerous flight behaviour (i.e. approaching close to power lines) in cranes Grus spp., mute swans Cygnus olor and other bird species.
- All markers except thin, black plastic strips or neoprene crosses were effective, with no differences in effectiveness between Bird Flight Diverters (BFDs: brightly coloured plastic spirals) and static fibreglass plates and only a small possible difference between BFDs and ‘flappers’ (moving markers).
Birds may fly into power lines accidentally, either because they do not see them at all, or because they see them too late to react; this is a particular issue for large species that cannot change direction quickly. Increasing power line visibility by marking them is therefore frequently proposed as a way of reducing collision-induced mortalities without the need to remove earth wires or bury transmission wires.
Supporting evidence from individual studies
A replicated, paired sites study (Morkill & Anderson 1991) in south-central Nebraska, USA, between 1988-1990, found that marking the static wire of nine spans of high-voltage transmission wire reduced collision mortality in sandhill cranes Grus canadensis by 66% compared with unmarked spans (11 fatalities vs. 25). Crane flocks were also more likely to increase altitude (454 flocks vs. 397) or change direction gradually (114 vs. 92) when flying close to marked spans, but were less likely to react quickly (and potentially dangerously) to marked spans (19 vs. 36) or show no reaction (1,200 vs. 768). Experimental spans were 1-2.5 km in length and marked with yellow aviation balls (30 cm in diameter with a vertical black stripe) at 100m intervals, staggered to appear more closely spaced.
A controlled before-and-after trial in the winters of 1989-90 and 1990-1 in mixed woodland and farmland in Extremadura, Spain (Alonso et al. 1994), found there were fewer collisions and fewer birds flying between lines following the marking of four sections of line (60% reduction in fatalities: 45 vs. 18; 61% decrease in birds flying between lines: 357 birds/day vs. 124 birds/day). There were no such decreases at unmarked spans of wire (19 vs. 15 fatalities). Markers were coloured PVC spirals (1 m long, 30 cm in diameter) attached every 10m.
A paired sites study in autumn and spring 1988-1991 in mixed wetlands, croplands and uplands in south-central Colorado, USA (Brown & Drewien 1995), found that collision mortality was 61% lower in four spans marked by ‘dampers’ and 63% lower in four spans marked by ‘plates’ compared with eight unmarked spans. Birds also reacted to marked lines earlier and flew over them at a greater height. ‘Dampers’ were yellow, spiral vibration dampers, 112-125 cm long, placed at 3.3 m intervals; ‘plates’ were yellow fibreglass plates, 30.5 x 30.5 cm with a 5 cm diagonal black strip, placed at 23-32 m intervals. Both markers were placed on two spans of 7.2 kV distribution lines and two of 69-115 kV transmission lines, with all spans totalling 13.2 km. Most birds killed were sandhill cranes Grus canadensis and wildfowl.
A paired sites study in 1991-4 in coastal wetlands in South Carolina, USA (Savereno et al. 1996), found a 53% reduction in collision mortalities at a 3.9 km span of 115 kV transmission lines where the static wires were marked, compared to a 1.2 km unmarked span. A higher proportion of birds approaching marked wires at the most dangerous height (between transmission wires and earth wire) reacted to them, compared to unmarked wires (98% of 9,819 flocks vs. 89% of 4,209 flocks respectively) and fewer crossed the wires at this height (4% vs. 24%). However, overall, a higher percentage of birds reacted to lines at unmarked spans (40% of 17,391 flocks vs. 34% of 64,512 flocks). The experimental span was marked with yellow aviation balls (30 cm in diameter with a vertical black stripe) at 61 m intervals, staggered to give the appearance of a 30.5 m spacing.
A controlled before-and-after study of three different markers at three sites in Spain in 1991-5 (Janss & Ferrer 1998) found that markers differed in their effects. In grassland and arable land in Badajoz, collision mortality across all species was 81% lower following the installation of white plastic spirals (expected mortality of 47 birds, actual mortality of nine). In mixed cattle grazing and cereal cropland in Cáceres, the total number of collisions (72) remained constant following the installation of neoprene strips, but there was a 76% reduction in collision mortality when great bustards Otis tarda were excluded from the analysis. In a wetland in Huelva there was no difference in collision mortality after the installation of black plastic strips (6 collisions in marked vs. 12 in unmarked spans). Plastic spirals were 1m long, 30 cm maximum diameter and placed every 10 m on static wires, staggered to give the appearance of a 5 m placement. Neoprene bands were crossed black strips, 35 x 5 cm with a 5 x 4 cm phosphorescent stripe, installed every 20 m on conductor wires and staggered to give the appearance of a 10 m placement. Black plastic stripes were 70 x 0.8 cm and hung every 12 m on a distribution line.
A literature review (Davis 1998) found three studies, all of which show a reduction in crane Grus spp. mortality, following the marking of power lines. Morkill & Anderson (1991) and Brown & Drewien (1995) are discussed elsewhere. The third study found that marking, burying or removing power lines in Japan reduced the percentage of red-crowned crane Grus japonicus mortalities attributed to power line collisions from 70.9% (1970-4) to 26.8% (1980-4). The rate of population increase rose from 5.9% (1970-4) to 18.9% (1980-4). There was also an increase in the number of breeding pairs and the percentage of juveniles in the population.
A controlled before-and-after study in 1997-2000 in northern Colombia (De la Zerda & Roselli 2003) found that the average number of bird deaths due to collision with power lines was significantly lower (5.3 deaths/ha) in a site where the ground wires were marked with yellow plastic spirals than for an unmarked circuit (13.6 deaths/ha); no significant difference in collision mortality between the circuits was detected prior to marking. A total of 810 birds of 47 species were found.
A before-and-after trial at a wetland site in Essex, England (Frost 2010), installed 500 red, spiral-shaped ‘flight diverters’ (32 cm long, 17.5 cm in diameter) at 5 m intervals along 1.5 km of power lines. In two springs preceding installation (2004 and 2006), 28 mute swans Cygnus olor were killed through collisions with the wires. Following installation, one swan was killed in the springs of 2007 and 2008 combined.
A replicated, controlled before-and-after trial over the winters of 2003-4, 2004-5 and 2005-6, in mixed wetland and crops on Staten Island, California, USA (Yee 2008), found a 60% reduction in collision mortality across all species following the installation of ‘Firefly bird flappers’ on 20 spans of a 5.6 km section of 12 kV wire. There was also a smaller (approximately 10%) reduction in mortality in nine spans adjacent to marked spans. There was no corresponding decrease in collision mortality in 20 unmarked spans. However, markers did not appear to have an effect on bird flight behaviour. Markers were 15 x 9 cm acrylic sheets of two contrasting colours with a luminescent strip and placed at 15 m intervals, staggered to appear 5m apart. A total of 65 fatalities were recorded over the three winters.
A 2010 literature review (Jenkins et al. 2010) found significant reductions in bird mortality following the marking of power lines in the USA and South Africa. A before-and-after trial over two years in Indiana (Crowder 2000), USA, found significantly reduced mortality with both Bird Flight Diverters (BFDs, 73% fewer fatalities) and larger ‘Swan Flight Diverters’ (50% fewer). The reviewers noted that there was considerable variability in collision rates across all sites. A before-and-after trial over three years in Karoo, South Africa (Anderson 2002) found a 67% reduction in collision mortality following the marking of both earth wires of 10 km of 132 kV line with BFDs (30 cm long) every 10 m. The same study also found that spans marked with BFDs and ‘flappers’ (loosely suspended polycarbon discs) had 52% lower collision mortality than spans marked just with BFDs, and 80% lower collision mortality than the same spans prior to marking. Spans with just flappers had 60% lower mortality than those with BFDs but the results are inconclusive. The reviewers noted a considerable decrease in blue crane Anthropoides paradiseus and Ludwig’s bustard Neotis ludwigii (the main species colliding with the line) populations in the area following marking.
Anderson, M. D. (2002) Large terrestrial bird powerline project. Unpublished report Eskom, Johannesburg.
Crowder, M. R. (2000) Assessment of devices designed to lower the incidence of avian power line strikes. Masters of Science Thesis, Purdue University, East LaFayette, IN. 91p.
- Morkill A.E. & Anderson S.H. (1991) Effectiveness of Marking Powerlines to Reduce Sandhill Crane Collisions. Wildlife Society Bulletin, 19, 442-449
- Alonso J.C., Alonso J.A. & Muñoz-Pulido R. (1994) Mitigation of bird collisions with transmission lines through groundwire marking. Biological Conservation, 67, 129-134
- Brown W.M. & Drewien R.C. (1995) Evaluation of two power line markers to reduce crane and waterfowl collision mortality. Wildlife Society Bulletin, 23, 217-227
- Savereno A.J., Savereno L.A., Boettcher R. & Haig S.M. (1996) Avian Behavior and Mortality at Power Lines in Coastal South Carolina. Wildlife Society Bulletin, 24, 636-648
- Janss G.F.E. & Ferrer M. (1998) Rate of bird collision with power lines: effects of conductor-marking and static wire-marking. Journal of Field Ornithology, 69, 8-17
- Davis C. (1998) A review of the success of major crane conservation techniques. Bird Conservation International, 8, 19-29
- De la Zerda S. & Roselli L. (2003) Mitigación de colisión de aves contra líneas de transmisión eléctrica con marcaje del cable de guarda. Ornitologia Colombiana, 1, 42-62
- Frost D. (2008) The use of 'flight diverters' reduces mute swan Cygnus olor collision with power lines at Abberton Reservoir, Essex, England. Conservation Evidence, 5, 83-91
- Yee M.L. (2008) Testing the Effectiveness of an Avian Flight Diverter for Reducing Avian Collisions with Distribution Power Lines in the Sacramento Valley, California: PIER Final Project Report. California Energy Commission report.
- 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