Photo by NorthWestern Energy.
500kV insulators heavily contaminated with bird droppings and pellets.

Montana Utility Manages Raven Roosts on Towers

Aug. 29, 2023
NorthWestern Energy shares three strategies to reduce faults from the growing raven population.

Parallel single-circuit 500-kV transmission lines in central Montana — jointly owned by NorthWestern Energy, Puget Sound Energy, Portland General Electric, Avista Corp. and PacificCorp — are an integral part of the Northwest power grid. These lines have experienced numerous zero-sequence, single-phase line-to-ground (S-L-G) faults of unknown origin since being energized in 1984. Research conducted along the lines from 2002 to 2010 determined many faults were likely caused by raptor streamers. Streamers are fluid feces forcefully ejected by raptors spanning many feet in length that are conductive and can bridge the air gap between conductors and towers.

More recent S-L-G faults on the 500-kV transmission lines from January 2017 to March 2017 differed from past events because they were sustained — relay reclose unsuccessful — and, therefore, of much greater concern and consequence to the service and stability of the grid. Additionally, on three occasions in 2017, both parallel lines faulted simultaneously. Initially, the faults were identified through the energy management system (EMS) by supervisory control and data acquisition (SCADA) processes. Further information obtained from NorthWestern Energy’s grid operations enabled the identification of the specific towers where the faults occurred. When crews visited the towers to investigate the sustained faults of unknown origin, they found insulators heavily contaminated with bird droppings.

The discovery of these fouled insulators confirmed the recent faults were not caused by raptor streamers. Moreover, the faults also occurred in late winter and early spring during dense fog or misty precipitation. These local weather conditions appeared to act as a catalyst, which suggested insulation breakdown as the reason for the faults rather than air-gap compromise caused by raptor streamers. Examination of event oscillograph outputs reflected that raptor streamer faults occurred indiscriminately across the voltage waveform, whereas contaminated insulator faults occurred at or near peak voltage. The outputs also revealed a distinct voltage rise (~35%) for approximately two cycles due to neutral shift on the un-faulted phases of the same circuit, as well as a similar rise (~20%) on the nearest adjacent circuit phase, which, if similarly contaminated with droppings, helped to facilitate the simultaneous fault.

Initial Strategies

NorthWestern Energy’s attempts to reduce faults from the accumulation of bird droppings over the last few years have involved several strategies.

First, tower crews cleaned contaminated insulators discovered during ground inspections following faults (that is, retroactive cleaning) and during routine transmission line maintenance flights (that is, preemptive cleaning). Depending on site accessibility, crews accessed towers by either climbing or using a bucket truck and cleaned insulators by hand or with a power washer, respectively. Crews could clean a maximum of only one tower per day when climbing and cleaning with hand brushes and two towers to three towers per day when using a bucket truck and pneumatic power sprayer. The sprayer was loaded with either pulverized corn cobs or walnut shells, media that effectively removed droppings without damaging the glass insulators. In late winter 2020, a helicopter-mounted, high-pressure water sprayer enabled crews to clean eight towers per day.

Second, tower crews replaced standard glass insulators with silicon-coated insulators on towers with the heaviest accumulation of droppings. Silicon-coated insulators historically have been used in marine environments to mitigate faults derived from salt accumulation and coastal fog. NorthWestern Energy believed coated insulators might also protect against contamination from bird droppings because of their physical properties: hydrophobic, higher-flashover voltage ratings and longer current leakage paths.

Identifying The Source

Curiously, the accumulation of bird droppings and number of non-streamer faults decreased during the summer of 2017, so the identity of the avian perpetrators remained unknown until late fall. In November 2017, while Northwestern Energy crews were finishing insulator washing late one afternoon as the sun began to set, hundreds of common ravens arrived from all directions to roost on towers for the evening. Finally, the utility had identified the birds responsible for causing the recent faults.

During the non-breeding season, ravens gather in groups to roost at night on trees, cliffs and anthropogenic structures such as towers, buildings and bridges. The single-night number of ravens at an individual roost can be impressive, approaching or exceeding 2000 birds. Raven roosts are typically seasonal, forming in the fall and disappearing in late spring as individuals disperse to breeding territories. This pattern of tower use coincided with the occurrence of NorthWestern Energy’s recent troublesome faults: most in the winter, fewest in the summer.

The size of the roosts on the 500-kV lines varied and consisted of two towers to 10 towers, depending on the location. Over the years, NorthWestern Energy has found seven large roosts on its transmission lines, spanning 110 miles (177 km) in central Montana.

A Third Strategy

After determining roosting ravens were the source of the problem, NorthWestern Energy’s third strategy was to install stainless-steel, avian perch deterrents to the lattice above insulators on towers within roosts. Spikes were approximately 6 inches (152 mm) in length and ordered in coiled strips of 100 ft (30 m). Spike strips were attached with screws to various-diameter polyvinyl chloride conduits, custom cut to fit tower members of differing sizes and configurations. Crews used metal zip ties to attach conduits to towers. From June 2018 to October 2019, crews installed deterrents on an average of four towers per day. To date, they have installed deterrents on 99 towers.

Crews installed and positioned perch deterrents to protect approximately half of each insulator string — a length of 126.5 inches (3213 mm) — from accumulating droppings. These arrangements effectively broke the leakage path of current responsible for faults and represented the utility’s working 500-kV hot-line gap settings of 50 inches to 55 inches (1270 mm to 1397 mm), adjusted for worksite elevation. By design, deterrents limited but did not completely exclude ravens from roosting on specific towers. Somewhat counterintuitively, NorthWestern Energy’s goal was to keep ravens on the towers they were using to discourage them from spreading to new towers, which would have created additional fault risks and mitigation efforts.
The three-pronged approach of washing insulators, installing perch deterrents and replacing glass with silicon-coated insulators has proven highly successful. The number of sustained faults declined from an average of 13 per year from 2016 to 2018 to an average of six faults per year from 2019 to 2021, and then dropped to only one fault in 2022. Although wind-dispersed raven droppings eventually contaminated the entire length of insulator strings on towers with perch deterrents during the winter, the rate of accumulation greatly decreased, reducing the time and effort needed for washing. Seasonal rains and declining roost sizes kept insulators free of contamination from spring to fall.

Why Ravens Now?

The 500-kV transmission lines have been in service for 40 years, so why did ravens only recently begin flocking to the towers for nocturnal roosting? Long-term data from the North American Breeding Bird Survey by the U.S. Geological Survey (USGS) show common raven abundance in central Montana during summer has significantly increased the last 10 years to 15 years. A combination of ecological factors at the landscape scale has likely facilitated raven population growth. For example, ravens are generalist feeders and readily exploit human-provided food subsidies such as cereal grains, landfill garbage and vehicle-killed deer, all of which have increased as native habitats have been converted by varying land uses.

Data from the National Audubon Society’s Christmas Bird Count show the winter raven population in central Montana is exhibiting exponential growth. This trend was statistically associated with the increasing number of faults annually from October to April before NorthWestern Energy took corrective actions. Therefore, continuing challenges to the operation of the 500-kV transmission lines should be expected. Existing roosts have the potential to become larger and spread onto previously unused towers as the wintering population increases. Ravens commute daily 15.5 miles to 34 miles (25 km to 55 km) one way to nocturnal roosts in other regions of the U.S. and Europe, so roosts along the transmission lines probably attract ravens from a wide geographic area in central Montana. Moreover, seasonal movements of ravens also play a role in roost formation, where individuals can migrate over 300 miles (483) from their summer breeding ranges to food-rich areas in the winter.

Strategies To Avoid

Over the years, NorthWestern Energy has considered but ultimately decided against hazing and shooting to reduce the size of raven roosts because these methods can illicit strong negative reactions from the public. More importantly, these methods have the additional potential drawback of dispersing ravens from established roosts to other towers, thereby spreading the risk of contamination and increasing the possibility for faults over a wider area.

The utility has found success with its three-pronged approach of washing insulators, installing perch deterrents and replacing glass with silicon-coated insulators.

James S. Lueck ([email protected]) has worked for NorthWestern Energy within the electric transmission department since 1975. He is currently the advisor for the 500-kV electric transmission operations. He holds an associate degree in electrical engineering technology.

Marco Restani ([email protected]) is a wildlife biologist for NorthWestern Energy. He is also professor emeritus at St. Cloud State University. His research and management program focuses on raptors and ravens. He holds a bachelor’s degree from the University of Montana, master’s degree from Montana State University, and a Ph.D. degree from Utah State University.

Editor’s note: More information about NorthWestern Energy’s efforts to reduce faults caused by raven roosts can be found in a peer-reviewed, scientific paper published in the journal Human-Wildlife Interactions (Restani and Lueck 2020, 14:451–460,

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