Effects of High-Voltage Power on Birds Breeding within the Powerlines' Electromagnetic Fields

Paul F. Doherty, Jr. and Thomas C. Grubb, Jr.

Abstract

Recently, the possible biological effects of electromagnetic fields (EMFS) produced by high-voltage transmission lines have come under intense scrutiny. Most of this attention has been focused on possible consequences for human health. Because very little has been done to assess the possible effects of EMFs on the biology of free-ranging animals living and reproducing within such fields, this study probed for EMF effects on cavity nesting birds.

We monitored the breeding biology of birds using nest boxes placed under three transmission lines and in three control areas. In Tree Swallows (Iridoprocne bicolor), there was a consistent trend across all three areas for reproductive success to be lower under the powerlines than in the control sites. No such effect was apparent in Eastern Bluebirds (Sialia sialis) or House Wrens (Troglodytes aedon).

Introduction

Recently, the possible biological effects of electromagnetic fields (EMFS) produced by high-voltage transmission lines have come under intense scrutiny. Results to date have been complex and inconclusive (Algers and Hennichs 1983, Nair et al. 1989, Tornqvist et al. 1991, Bernhardt 1992). Many studies probing for EMF effects have produced negative results, but there is a growing record of studies showing EMF effects on the cellular, whole-animal and epidemiological levels. Most of this attention has been focused on possible consequences for human health (Nair et al. 1989).

According to the U.S, Department of Energy, there are currently 347.200 miles (560,000 km) of high-voltage transmission line, and 3.2 million km of lower-voltage distribution line in service in the United States (Minner 1987). Very little is known about possible EMF effects on free ranging animals near these lines. Orbrecht et a/. (1991) suggested that areas beneath high-voltage power lines may be good conservation habitat in urban areas. Published research accounts deal mostly with the effects of habitat change caused by clearing transmission line corridors on the species composition and densities (Anderson 1979, Kroodsma 1982, Niemi and Hanowski 1984) and with audible noise caused by transmission lines (Lee and Griffith 1978). Electromagnetic fields produced by a 500-kv powerline in northern Minnesota did not affect bird densities as compared to nearby control areas (Niemi and Hanowski 1984), nor did EMFs produced by an extremely low frequency antenna system in Wisconsin affect the abundance of species richness of breeding and migrating birds (Blake et aL 1990).

None of the many studies relating EMF to field crops, meat production, or milk output has shown any effect (Morgan 1989). Some migratory birds and fish have been shown to use naturally occurring electromagnetic fields as a cue for navigation. Although no specific manipulative tests have been done, there is no evidence that man-made fields around powerlines disrupt reception of such cues (Morgan 1989).

In the few animal populations where reproductive output within EMFs has been monitored, no effects have been shown. Gilmer (1977) found only slight, non-significant differences between the productivity of Ferruginous Hawks (Buteo regales) nesting on 230-kv transmission line towers and that of hawks nesting on other substrates, while Steenhoff et a/. (Steenhoff and Kochert 1993) found no difference in nesting success between various raptors or Common Ravens (Corvus corax) nesting along a 500-kv line and those nesting away from the powerline.

The objective of our study was to probe for effects associated with high voltage transmission lines on the reproductive biology of cavity nesting birds breeding directly beneath the powerlines. Our approach was to compare treatment groups situated beneath powerlines with nearby control groups that were out of the range of electromagnetic fields.

Methods

Treatment sites were selected directly under the midline of a 765,000-volt transmission line in Alum Creek State Park (Delaware County, Ohio), a 69,000-volt transmission line in Caesar Creek State Park (Warren County, Ohio) and a 69,000volt transmission line near the city of Tiffin (Seneca County, Ohio). At each location, a control site was established nearby, but out of the electromagnetic field. The control site was chosen to match the treatment site in vegetation structure, plant species composition, and proximity to water (Doherty 1994). At each of the six sites, nest boxes were erected 11 to 38 ft (10 to 35 m) apart in a line and were monitored throughout the breeding season of 1993.

At all study sites, clutch size, number hatched, and number fledged were recorded, as well as electric and magnetic field strengths at each box. We compared treatment and control groups at all three locations combined, using first the nest box and then, to increase the scope of statistical inference, the location as the primary sampling unit. When the nest box was the primary sampling unit the scope of statistical inference was limited to all the birds that could have bred within the particular boxes we used. For this analysis, we used a Two-way ANOVA with unequal sample sizes to test for differences among location (Alum Creek, Caesar Creek, and Tiffin) and between treatment (under a power line or not) and for any interaction between the two (GLM - Minitab, 1993). When the location was used as the primary sampling unit the scope of statistical inference was expanded to birds breeding under high-voltage power lines. For this latter analysis, we used two-sample t-tests and performed power analyses to determine the minimum sample sizes required to detect statistically significant (a = 0.05, fl = 0.05) differences between power line and control areas.

Results

We found no effect of transmission lines on any measure of reproduction in either Eastern Bluebirds (Sialia sialis) or House Wrens (Troglodytes aedon) (Tables 1 and 2). However, in Tree Swallows (Iridoprocne bicolor), reductions in breeding success under the power line occurred at all three study sites. When the nest box was considered the primary sampling unit, pairs of Tree Swallows under power lines produced significantly fewer fledglings (Table 3), resulting in significant reductions in percentage of nestlings fledged and overall reproductive success (percentage of eggs fledged, Table 3). No significant interactions between location and treatment were detected. The statistical inference to be drawn from these results extends only to the set of Tree Swallows that could have nested in the particular boxes we monitored, but the scope of biological inference may be greater. To widen the scope of the statistical inference and to strengthen the biological inference, the data were also analyzed using the site as the primary sampling unit. No statistically significant effects could be found with these small sample sizes (Table 3), although the difference in percent fledged would have been significant had we employed a less conservative one-tailed test. Power analysis suggests that sample sizes from 6-9 would have caused results from our two-tailed tests to reach statistical significance (Table 4).