The Price of Power: Understanding the Effects of Power Lines on Birds

Linear barriers such as power lines, right-of-way clearings, pipelines, seismic lines and even fences and ditches are ubiquitous features across our landscape and can pose significant threats to ecosystem integrity and at-risk species. Of these disturbances, power lines pose the greatest threat to avian populations. Mortalities from collisions with power lines and electrocutions on poles are well-documented (for review see Jalkotzy et al. 1997). Although the total number of these mortalities is relatively low compared to road kill (Janss and Ferrer 1998), even low mortality levels can have a significant effect on at-risk populations. Recent research has shown that bird morphology and behavior can determine which birds are at greatest risk of being killed. Additionally, visibility and type of wires and poles are all factors in bird mortality rates. This article summarizes recent literature on the effects of power lines, poles, and electromagnetic fields, and concludes with a list of proposed mitigation measures.

Review of Literature

Power Lines

Ornithologists studying bird/power line interactions have long hypothesized that bird morphology plays an important role in the likelihood of collision (Jalkotzy et al. 1997). Building on this research, a recent study in west-central Spain attempted to establish a model for determining species' risk of mortality from power lines. Over the course of two years, species abundance was measured in three habitat types, and road surveys were conducted along power lines to determine the relative abundance of raptors, ravens and storks, the three bird species that other studies have identified as being most at risk of electrocution. In addition, several mortality surveys were conducted beneath power poles in the region during roughly the same time period. The results from the two surveys were pooled, documenting a total of 41 species killed by power lines. Overall, species with a greater wing loading value (a greater weight-to-wing ratio; ducks vs. herons, for example) were more likely to die of power line collisions than from electrocutions (Janss and Ferrer 1998).

Several studies have attempted to determine the efficacy of power line modifications designed to make wires more visible to avifauna or to otherwise protect them from power line collisions. During a three- year monitoring study Savereno et al. (1996) compared avian reaction to two transmission lines (with marked and unmarked static/grounding lines) running through saltmarsh along the South Carolina coast. Thirty-five to forty percent of the flocks exhibited behavior change in reaction to both line types. Of the flocks approaching at line height, 98% changed behavior at the marked site, compare to 89% at the unmarked, indicating the markers did have an effect. Flocks also tended to approach the marked line at lower elevations than at the unmarked line. Observed collisions at both sites were primarily with the static line. A similar study compared ptarmigan collisions between three sections of power line over a six-year period (Bevanger and Broseth 2001). On one of the sections, the static line was removed, and a 51% reduction in collisions resulted. In a third study, white wire spirals and black bands crossed to form an "x" reduced overall collision mortality by 75%, although the Great Bustard, an endangered species that suffers significant collision mortalities, did not respond to the markers (Janss and Ferrer 1998).

A study conducted in Spain compared avian mortalities on three different types of power lines: one transmission wire with static wires, and two smaller distribution lines, each with three wires on virtually the same horizontal plane (Janss and Ferrer 1998). Each type of line was at a significantly different height. The lowest of the distribution lines experienced much higher collision frequencies. Because no general bird abundance surveys were conducted, clear conclusions cannot be drawn regarding type of line and collision frequency, except to note that distribution lines, which are less studied than transmission lines, can certainly be as significant a cause of mortalities.

Power Poles

While power lines pose a number of threats to a variety of birds, the poles that support them are most likely to affect raptors and corvids. These birds are most at risk due to their relatively wide wingspans and tendency to use poles as nesting platforms and perches from which they survey for prey (Lehmann 2001). Studies have shown that golden and bald eagles suffer some of the highest mortalities; one study based on ten years of data collected from 13 western states and Canada found that out of 1,450 confirmed raptor electrocutions, 272 were golden eagles. Electrocution deaths occur when a bird's body bridges the gap between two energized components, most likely the wires (Harness and Wilson 2001). Studies generally rely on ground surveys beneath poles and necropsies or visual signs (scorch marks) to confirm electrocution deaths, but this technique may only provide a low estimate of electrocution mortalities (Deng and Frederick 2001). Reporting of raptor electrocutions is not required of utilities, and since raptor deaths are a violation of federal law, there may be a considerable disincentive to reporting.

In some areas raptor and corvid populations may benefit from the presence of power poles. In a study of a 596 km segment of transmission line through southern Idaho and Oregon, researchers found that over the course of nine years, nesting pairs of raptors and corvids on the steel transmission towers increased from three to 133. In the same study, nesting success for golden eagles was ten percent higher for nests on the transmission line than in cliffside nests in the area population (Steenhof et al. 1993).

Electromagnetic Fields

There is growing concern that the strong electromagnetic fields (EMFs) generated by wires may adversely affect the health of avian populations. One study (Fernie et al. 2000) observed two groups of mated American kestrels under controlled conditions, one exposed to EMF levels comparable to a transmission line. Pairs in the EMF-exposed group had significantly higher fertility rates, larger eggs and embryos, and higher fledgling success. However, EMF-exposed eggs had significantly thinner shells and a lower hatching success. The reduced hatching success may be related to both the thinner shells (increased porosity) and the larger embryo size, which may reflect EMF effects on thyroid development. Doherty and Grubb (1998) examined the nesting success of birds out in the field under several power lines and found species-specific effects. EMF exposure appeared to have little effect on house wrens nesting under transmission lines. However, comparing EMF-exposed tree swallows with control populations revealed that EMF-exposed swallows had significantly wider eggs, with greater volume, and lower fledging success, leading to an overall lower rate of reproductive success.

Recommendations

While power lines and poles pose a potential danger to birds, there are a number of ways in which this threat can be mitigated, if not eliminated. The following are recommendations inferred or taken directly from the previous papers.

1. Remove static lines from poles. While these do serve to stabilize high-voltage lines, officials in Norway have admitted that at voltages below 132kv, their utility is less than certain (Bevanger and Broseth 2001). If they are proven to be unnecessary, doing away with them could prevent hundreds of collision deaths.

2. Use wire markers on static lines. Though effectiveness varies by species, wire markers should help reduce collisions.

3. Consider the presence of species of concern when choosing power line locations. Some species, like the Great Bustard in Europe, cannot afford power line casualties. Avoiding the division of breeding and feeding grounds by lines could cut down on fatalities, as could burying the lines or locating them all on the same horizontal plane.

4. Minimize opportunities for birds to come in contact with wires. Place perches above wires, ensure that wires are spaced to accommodate the wingspan of the largest birds, and provide nesting platforms in areas where raptors are likely to nest on poles. Wooden poles also help reduce the number of electrocutions.

5. Create a centralized reporting system. Utilities should be required to report avian mortalities in a standardized fashion. Doing so would provide a wealth of information about avian deaths, and would provide a means for tracking down the worst offenders.

Conclusion

While the era of decentralized electric generation may be on the horizon, it's unlikely that we will do away with the infrastructure of the energy grid anytime soon. However, practicing responsible site placement and more avian-aware design with both new and existing power lines and poles will go a long way toward protecting and preserving avian populations and the ecosystems of which they are a part.

--- Katharine Hyzy is a graduate student in the Environmental Studies program at the University of Montana, and has worked on energy issues in Montana and Oregon.

Biliography

Bevanger, K. and H. Broseth. 2001. Bird collisions with power lines-an experiment with ptarmigan. Biological Conservation 99: 341- 346.

Deng, J. and P. Frederick. 2001. Nocturnal flight behavior of waterbirds in close proximity to a transmission power line in the Florida Everglades. Waterbirds 24(3): 419-424.

Doherty, P. and T. Grubb. 1997. Reproductive success of cavity- nesting birds breeding under high- voltage power lines. American Midland Naturalist 140: 122-128.

Fernie, K., D. Bird, R. Dawson and P. Lague. 2000. Effects of electromagnetic fields on the reproductive success of American kestrels. Physiological and Biochemical Zoology 73(1): 60-65.

Harness, R.E. and K.R. Wilson. 2001. Electric-utility structures associated with raptor electrocutions in rural areas. Wildlife Society Bulletin 29: 612- 623.

Jalkotzy, M.G., P.I. Ross and M.D. Nasserden. 1997. The effects of linear developments on wildlife: a review of selected scientific literature. Prepared for the Canadian Association of Petroleum Producers. Arc Wildlife Services Ltd., Calgary.

Janss, G. and M. Ferrer. 1998. Rate of bird collision with power lines: effects of conductor-marking and static wire-marking. Journal of Field Ornithology 69(1): 8-17.

Lehman, R.N. 2001. Raptor electrocution on power lines: current issues and outlook. Wildlife Society Bulletin 29(3): 804-813. Savereno, A., L.

Savereno, R. Boettcher and S. Haig. 1996. Avian behavior and mortality at power lines in coastal South Carolina. Wildlife Society Bulletin 24(4): 636-648.

Steenhof, K., M. Kochert and G. Roppe. 1993. Nesting by raptors and common ravens on electrical transmission line towers. Journal of Wildlife Management 57(2): 271-281.