Effectiveness of Raptor Perch Deterrents on an Electrical Transmission Line in Southwestern Wyoming

ABSTRACT In sagebrush—steppe and other open habitats, power lines can provide perches for raptors and other birds in areas where few natural perches previously existed, with potential negative impacts for nearby prey species, such as greater sage-grouse (Centrocercus urophasianus). Between September 2006 and August 2007, we used driving surveys, behavioral-observation surveys, and prey-remains surveys to assess the ability of perch-deterrent devices to minimize raptor and common raven (Corvus corax) activity on a recently constructed transmission line in southwestern Wyoming. All survey methods demonstrated that activity was significantly lower on the deterrent line compared with a nearby control line; however, deterrent devices did not entirely prevent perching. Considering use of cross-arms or pole-tops alone, we sighted 42 raptors and ravens on the deterrent line and 551 on the control line during 192 driving surveys of each line. Golden eagles (Aquila chrysaetos) and ravens were the species most commonly observed successfully overcoming deterrent devices. Smaller rough-legged hawks (Buteo lagopus) regularly avoided deterrents by perching on conductors (i.e., wires). We documented much off-line activity near both survey lines and suggest that fewer birds near the deterrent line likely reflected reduced availability of nearby alternate perches. There was a pronounced winter peak in on-line perch use, with the effect more evident on the control line. Behavior surveys corroborated our driving-survey results but were otherwise unproductive. During 549 prey-remains surveys of each line, we found 9 single and 60 grouped prey items near deterrent-line poles, compared with 277 single and 467 grouped items near control-line poles. We observed few sage-grouse in the study area but did witness a likely power line—related, raptor-caused sage-grouse mortality. Overall, our results suggest that perch-deterrent devices can reduce raptor and raven activity on power-line structures, but to determine their utility on entire power-line segments, we suggest managers consider 1) what level of reduction in perch activity is worth the cost, and 2) the availability of alternate perches in the surrounding landscape.     

Effectiveness of Avian Predator Perch Deterrents on Electric Transmission Lines

ABSTRACT A new high-voltage transmission line in north-central Nevada, USA, was considered a potential threat to greater sage-grouse (Centrocercus urophasianus) because avian predators are attracted to and hunt from elevated perches. As a mitigation measure, perch deterrents were installed on the transmission line towers at the time of construction; in addition, 2 existing high-voltage transmission lines were retrofitted with deterrents. Previous published studies have investigated the efficacy of perch deterrents in preventing or reducing electrocution of avian predators and fecal contamination of insulators, but none have evaluated deterrents as a means of eradicating perching on towers. We conducted point transect surveys and perching-duration observations of corvids and raptors and determined that although perch deterrents did not prevent perching, the perching duration of raptors on the deterrents was reduced compared to other perching substrates. Perching of raptors indicated that some hunting most likely took place from the towers; therefore, the deterrents did not completely obviate the threat that avian predators posed to greater sage-grouse. Although the deterrents reduced the probability of avian predators perching on the towers, avian predators overcame the deterrents to take advantage of the height of the towers where no other perches of similar height existed. The perch deterrents as designed did not have the desired short-term effect on avian predators, but further monitoring may reveal longer-term effects and distinguish perching behaviors specific to different species of avian predators.