Study Area

We conducted our study in the state of Rajasthan, India (Fig. 2), between the cities of Bikaner (28°1.2′N, 73°19.08′E) and Jaisalmer (26°54.9′N, 70°54.5′E). Bikaner and Jaisalmer are located in the Thar Desert, where the average annual rainfall is 10–40 cm and temperatures range from a minimum of −2°C in winter to a maximum of 51°C in summer (Sharma and Mehra 2009). Vegetation is mainly dry, open grassland or grassland interspersed with thorny, drought-resistant bushes growing in sandy soils (Gupta 1975). In arid areas where tall trees are largely absent, anthropogenic structures such as utility poles provide tall perches used by a variety of birds, including raptors (Benson 1981, Dhindsa et al. 1988).

Figure 2.

Locations of line segments surveyed from 31 November through 6 December 2011 in Rajasthan, India. Numbers in the figure indicate voltage (kV) of the line segments.

We selected this area because 98% of rural villages were electrified as of 2004 (Modi 2005). Rajasthan also contains sites identified as high-priority areas for avifaunal conservation, and is recognized by the Bombay Natural History Society of Mumbai as an International Birdlife Area (Islam and Rahmani 2004). The area supports an estimated 250 avian species (Rahmani 1997) and includes the rural village of Jorbeer, where a regional disposal location for cattle carcasses exists. Religious practices preclude human consumption of cattle in the study area. As a result, cattle carcasses are skinned for leather and otherwise discarded intact near Jorbeer. The carcass disposal site attracts numerous wintering raptors (Sharma and Sundar 2009), including four eagle species (Eastern Imperial Eagle [Aquila heliaca], Indian Spotted Eagle [A. hastata], Steppe Eagle [A. nipalensis], and Tawny Eagle [A. rapax]) and six vulture species (Cinereous Vulture [Aegypius monachus], Egyptian Vulture [Neophron percnopterus], Eurasian Griffon Vulture [Gyps fulvus], Long-billed Vulture [G. indicus], Red-headed Vulture [Sarcogyps calvus], and White-rumped Vulture [G. bengalensis]). Three of the six vulture species noted in our study area are of conservation concern, with the Egyptian Vulture listed as endangered, the Red-headed Vulture listed as critical, and the Cinereous Vulture listed as near threatened (Thakur and Narang 2012). Vulture numbers have been declining in India (Prakash et al. 2005, Sharma 2012) and vulture electrocutions are a persistent problem globally (Anderson 2000). Thus, information on compounding factors such as electrocution, is critical to the effective management and conservation of this taxonomic group.

Survey Methods

From 31 November through 6 December 2011, we drove Highway 15 from Jaisalmer to Bikaner in search of rural power lines with public access. Restricted by the terrain, we visited 13 energized line sections and two nonenergized line segments in rural habitat parallel to public roads, but away from towns and villages. In the Discussion, we address the potential consequences of using convenience sampling focused on areas of public access rather than randomized sampling regardless of ownership. To conduct surveys, two of us walked the entirety of each sampled line segment, inspected each pole-top, and searched the ground within a radius of 7.6 m around the base of each pole for the presence of avian remains (Harness 2001, Dwyer and Mannan 2007). We also walked beneath the wires to record mid-span fatalities that may have resulted from power-line collisions.

We recorded six variables at each pole: conductor separation, insulator configuration, jumper count, line voltage, surrounding habitat, and whether or not an avian carcass was present. We identified the presence of an avian carcass a priori as our response variable for both univariate analyses and multivariate modeling.

We classified conductor separation as either sufficient or insufficient, following APLIC (2006) guidelines. APLIC (2006) suggests that to prevent avian electrocution, conductors should be separated from one another by at least 152 cm horizontally and 102 cm vertically (Fig. 3). If phase-to-phase and phase-to-ground separation equaled or exceeded APLIC recommendations, we recorded the pole as having sufficient separation. If not, we recorded insufficient separation. This level of separation is designed to prevent the electrocution of large eagle species in North America (e.g., Golden Eagle [Aquila chrysaetos]), because meeting separation requirements for eagles acts as an umbrella to minimize risk for smaller species. We address extrapolation to Indian species in the Discussion.

Figure 3.

Examples of risk points where sufficient conductor separation is required to minimize avian electrocution risk on an energized utility pole: (A) phase to grounded crossarm, (B) phase to grounded pole top, and (C) center phase to outer phase.

We classified the insulator configuration on each pole as low, high, or absent (Fig. 5). Each pole carried three phase wires, with the center phase typically supported on top of the pole by a pin insulator. If the top of the center-phase insulator was ≤30 cm above the grounded bracket, we recorded a low configuration. If the top of the center-phase insulator was >50 cm above the grounded bracket, we recorded a high configuration. We estimated insulator heights based on known pole-top dimensions. If the phase wires were suspended rather than supported by pin insulators, we recorded absent as the configuration.

The jumper count equaled the number of jumpers on each pole (Fig. 4). Jumpers are short wires that connect energized devices such as switches and transformers to the primary circuit wires running between poles. Jumpers are also used to connect primary wires at poles where lines diverge or change direction. Counting the number of jumpers allowed us to incorporate all energized equipment in a single modeled variable. This avoided problems with lack of independence among variables, which otherwise would have arisen if we quantified each type of pole-mounted equipment separately (Dwyer et al. 2013).

Figure 4.

Indian Roller (Coracias benghalensis) perched on a typical 11-kV utility pole with six jumpers in Rajasthan, India.

Figure 5.

Illustration of low (A; ∼30 cm) and high (B; ∼50 cm) insulator configurations on a utility pole.

We classified line voltage as 0 kV (nonenergized), 11 kV, or 33 kV. Because larger insulators are required for higher voltages, we used insulator size to distinguish between voltage levels. Nonenergized sections included either poles with no wires or poles with wires that were not energized. Nonenergized and energized poles were configured identically, and we surveyed both types in the same manner. We visited nonenergized poles, where birds could not possibly have been electrocuted, because an avian carcass found dead beneath a power line is not necessarily the result of electrocution (Dwyer 2004). This is particularly true in India, where diclofenac poisoning of vultures has been a persistent concern (Oaks et al. 2004, Schultz et al. 2004, Prakash et al. 2005). Consequently, identifying electrocution as the cause of death may reflect researcher bias, unless assumptions of electrocution are verified. We used the number of carcasses found (n  =  1) beneath nonenergized poles (n  =  78) to estimate the proportion of carcasses beneath energized poles not attributable to electrocution.

We classified habitat as disturbed or undisturbed. We defined disturbed habitats as those with anthropogenic features such as buildings or roads within 50 m of the pole, and undisturbed habitats as lacking those features within 50 m.

For each surveyed pole, we recorded whether one or more carcasses was present, and if present, the number of carcasses, the species of each carcass, and whether there was evidence of electric-shock injury. We used presence-absence data as the response variable in logistic regression models. Many carcasses were decomposed or desiccated, thus precluding formal necropsy. Instead of necropsies, we followed Dwyer (2004, 2006) and Harness and Hurmence (2004) in identifying external damage (burns) to carcasses consistent with electrocution (Fig. 6). We left all carcasses where we found them and used a GPSmap 62s (Garmin, Olathe, Kansas, U.S.A.) to record carcass locations. Carcass locations are available upon request so future researchers can avoid identifying known carcasses as new records.

Figure 6.

Eurasian Kestrel (Falco tinnunculus) with electric burn marks visible on the left scapulars, found beneath an energized utility pole in Rajasthan, India.

Statistical Methods

Because we surveyed every accessible pole, we treated poles as independent samples, even though they occurred in distinct line segments. A very low number of carcasses found under nonenergized poles (n  =  1) obviated the need for statistical analysis to discern differences in the probability of finding a carcass under energized versus nonenergized poles (see Results). This fact and burns observed on carcasses supported the assumption that electrocution was the cause of death for most individuals found under energized poles. We used multivariate logistic regression to model the probability that at least one avian carcass was found at the base of an energized pole, with electrocution the presumed cause of death. We used a logit link in the gmulti package (Calcagno and de Mazancourt 2010) for program R (The R Foundation for Statistical Computing, Vienna, Austria) to model all possible subsets of the five candidate predictor variables (n  =  32 candidate models), to rank models using Akaike's Information Criterion (AIC), and to calculate an averaged model, averaged model parameters, and standard errors for averaged model parameters based on the response variable (presence-absence of a carcass). We also used the binary logistic regression option in Minitab 16 (Minitab, State College, Pennsylvania, U.S.A.) to conduct the Hosmer-Lemeshow (Hosmer et al. 2013) goodness-of-fit test on the global model with no interactions. The goodness-of-fit test supported use of AIC corrected for small sample size (AICc; Anderson 2008) to rank models (  =  0.778, df  =  3, P  =  0.855).

Out-of-sample cross validation uses individuals of interest (power poles in this case) not used in model building to test the predictive ability of a model (as in Tintó et al. 2010). We used 80% of the data from energized poles (478 poles) to create the model and the remainder (119 poles) to compose an out-of-sample cross validation test. Parameters with standard errors (SE) overlapping zero did not contribute substantially to the overall fit of the final model. We do not report these parameters as part of the final model and excluded them during model validation.

Management Implications

There is an ongoing, concerted effort to provide power to all rural areas in India (Modi 2005). Power lines can play a positive role for raptors by providing nest substrates and perches used for hunting and roosting (APLIC 2006, Puzović 2008, Ellis et al. 2009). As demonstrated here, however, the trend to use concrete poles with metal cross-arms counters at least some of the potential benefits for birds. Existing poles, particularly those with a modeled electrocution risk of ≥20%, should be retrofitted and new poles should be constructed to standards explicitly designed to minimize avian electrocution risk. This is particularly important near the Jorbeer carcass disposal site, where we noted numerous vultures around recently constructed and presently nonenergized power lines. If these lines are energized without first installing conductor covers, substantial numbers of avian electrocutions are likely.

To prevent avian electrocutions, APLIC (2006) recommends 152 cm of horizontal and 102 cm of vertical separation between different phases and between phases and grounded equipment, including concrete poles and grounded metal cross-arms. These parameters are useful to electric utility personnel trained in the technical details of power poles, but untrained observers may have difficulty discerning such characteristics. Critical dimensions defined by APLIC (2006) also may fail to protect the Cinereous Vultures and Eurasian Griffon Vultures occurring in our study area, which can exceed the size of Golden Eagles in North America by 4–22% (Ferguson-Lees and Christie 2001). Using jumper counts and insulator configurations as predictive variables facilitates quantification of avian electrocution risk without requiring detailed understanding or specialized training to distinguish different types of pole-mounted equipment, and without relying on separation standards developed outside India. With the simple model we provide, personnel with minimal training in electric utilities can report the results of surveys quantifying the jumper count and pin height on poles, and utilities can subsequently prioritize retrofitting the most dangerous poles. To do so, utilities only need to use the following two equations, entering field data into Equation 1 and the output from Equation 1 into Equation 2.

In Equation 1, the jumper count is entered as a numerical value from 1 to n. For the other two variables (pin insulator and insulator height) only categorical values of 0 or 1 are allowed in the equation. If phase wires are suspended such that no pin insulator exists, then pin insulator  =  0 and insulator height  =  0. If a pin insulator exists, then pin insulator  =  1 and the insulator height value depends on the configuration. If a high pin insulator exists, then insulator height  =  0. If a low pin insulator exists, then insulator height  =  1. If primarily large species are being considered, such as near the Jorbeer carcass disposal site, then use of a value of 1 for the insulator height variable will likely yield the greatest benefit, regardless of actual pin insulator heights.

The least expensive strategy to minimize avian electrocution risk on new construction is to frame poles with sufficient spacing between conductors or conductors and grounds to prevent simultaneous contact (APLIC 2006). When using concrete poles, alternative construction methods must be used and framing changed to prevent electrocutions (Harness 1998, Cartron et al. 2004). In Russia, new steel-pole lines are being constructed using suspension insulators, which apparently is a reliable configuration that allows for increased span lengths (Hunger et al. 2006) and safely accommodates perching by both small and large birds. Retrofitting existing electric poles is more expensive, but can be accomplished by insulating or isolating energized equipment. Specific to the poles evaluated here, insulation should be added to all jumpers, all equipment serviced by those jumpers, and all conductors on metal or concrete substrates. If even a small gap remains on a jumper, electrocution risk can remain high and avian electrocutions can persist (Dwyer and Mannan 2007).