Seekiefer (Pinus halepensis) 9months-fromtop.jpg
This conservation resource was created by Jingyu Guan, Jason Henry, and Yehuda Huberman.

Raptor nests on power poles: advantages and disadvantages

A map of the high voltage transmission lines in the United States, data source from FEMA via NREL. Rolypolyman at English Wikipedia. Public domain.

Raptor use of power-line structures has been documented globally for activities such as nesting, roosting, hunting, feeding and establishing territorial boundaries [1] [2]. This creates challenges and opportunities. One challenge is that electrocution of birds on power-line structures is thought to be a cause of decline in raptor populations in some areas, including in populations of threatened species[2]. Other interactions between raptors and power-line structures may be economically costly in the form of power outages to vast areas, even if the birds themselves are unharmed [3]. On the other hand, use of man-made structures has increased habitat for raptors in areas where natural perch sites are limited. This has helped populations of rare or threatened birds increase [3]. In fact, increasing raptor populations due to these structures is a cause of concern in certain areas where increased predation threatens other species [4] [5]. These opposing effects of population increase and decrease is due to variability in the configuration of power-line structures and to the way different bird species interact with them. Therefore, whether the use of power-line structures by raptors should be encouraged or discouraged – and what tactics should be used to achieve those goals – depends fully on the details and context of each case.

Contents

Background

Raptor is a common name for diurnal birds of prey of the order Falconidae. They are characterised by keen vision, powerful flight, and strong talons and beaks. Size can range from a wingspan of only 50 cm, as with the American kestrel, to over 200 cm, as with many eagles. Some of the representative species include osprey (Pandion haliaetus), common kestrel (Falco tinnunculus), Northern goshawk (Accipiter gentilis), red kite (Milvus milvus), common buzzard (Buteo buteo), peregrine falcon (Falco peregrinus) and bald eagle (Haliaeetus leucocephalus). They are found all over the world. The increasing urbanisation and industrialisation in many parts of the world have caused an increase in power lines and structures, with which birds often come into contact.

Issues for individual raptors

Raptors often build their nests on power poles. This creates risk for the electrocution of both the adults and the young. [6] [7] [8]. Many raptor species, such as the Mauritius kestrel (Falco punctatus), are already an endangered or threatened species based on the IUCN list of Threatened Species and the threat of electrocution only adds to this conservation concern [9].

Ecological concerns

Studies have shown that the raptor is a predator at the top of the food chain [10]. This means that a decrease in the raptor population will have repercussions throughout the food web as the prey increase and consume the level below it. In the long run the ecosystem will be affected negatively through large fluctuations of an unnatural population cycle [11].

Public concerns

When a nest is made on a power pole, there are a number of potential problems that can occur. The first is power outages. When raptors are electrocuted on power lines a short in the electric circuit causing power outages can be created. An estimated 25% of all power outages in the United States are caused by birds [12]. In South Africa’s Karoo region, excrement from nesting eagles cause short-circuits in the network leading to costly power outages, although the birds are unharmed [3]. In this case, relocating nests to below the conducting elements while placing perch deterrents above them reduced faulting by over 60% [3]. The cost of this project amounted to US$120,000, while the power companies saved US$260,000 from reduced outages [3].

In contrast, on power lines in southern Idaho only 3% of golden eagle and hawk nests were built directly above insulating strings and therefore the risk of nest material or excrement contaminating the insulation and causing power outages was low. Nest sites were usually found on sections with dense steel latticework. Removing insulation from directly below those sections would therefore remove any remaining concern. As raptors tend to return to the same areas to nest, this modification would have to be made only for towers known to be used as nest sites. On the other hand, data on nest persistence and rates of nest re-building show that removing nests would not act as a successful deterrent [13].

A second problem is that the short in the electric circuit or debris falling from the nest can cause fires. The cost to the electric company of repairing the lines, in addition to the cost in human resources and time, is another concern. In BC, the osprey is one of the common raptor species that electric companies have to deal with as they tend to build big, heavy nests on power poles. They also like to find the tallest perch from which to get the best view for hunting. Electricity company like FORTIS BC have an Osprey Nest Management Program whose task is to properly manage the osprey nests. [14]. In BC, Ospreys are a protected species, and permits are required to relocate their nests. When this happens, an alternative pole is set up for them which requires resources [15].

Electrocution

A common distribution pole in North America responsible for a large number of avian deaths. Andrew Schmidt. Public domain, via PublicDomainPictures.net

The electrocution of raptors is a problem that exists throughout the world. For example, in Spain, electrocution is the leading cause of death in the Spanish imperial eagle and is a significant cause of death of many other raptors in the area as well [16]. Bird electrocutions on overhead wires has been a problem known since the advent of electricity production. Despite this long running knowledge of the problem, there is little concerted effort being focused on the issue, with a large portion being performed by power companies on their own initiative. In the United States, bird deaths from man-made causes were first estimated in 1979 at 196 million bird deaths and in 1995 electrocution as the 2nd highest cause of death in golden eagles and the 3rd highest in bald eagles. These estimates are believed to be a very poor representation as it is believed that deaths are many times higher due to an inability to monitor the large amount of remote transmission lines in the United States [17]. The estimation of death is further complicated by the scavenging or consumption of dead raptors around power poles, and raptors that die of other means while on the power pole and make a connection between wires on the way down [1]

Cause and effect

The electrocution of raptors and other birds on powerlines is caused when the bird creates a bridge between two live conductors or a live conductor and the ground. This causes the electricity to follow the newly created path through the bird. This electric current can either cause an electric shock to the bird in the case of relatively low power and a large bird, or can result in electrocution in most cases. The reason that this affects raptors to such an extent is due to the habits of most raptors to use power poles for multiple purposes including nesting and feeding, and the generally longer wingspan compared to most other birds. This is notable as more electrocutions occur on lower voltage lines (<69kV) compared to higher voltage lines, this occurs because the insulated clearance distance is larger on the higher voltage lines, and larger than the wingspan of most raptors. This risk is increased through multiple factors, including the habits of raptors, landscape influences, and pole design. A summary of multiple studies estimating electrocution deaths on power poles has a large variation from 0.0059 birds/pole/year to 0.359 birds/pole/year, the greatest factor in this large variation is due to the main material used to construct the tower with more conductive materials having higher rates. When reduced to only wooden poles which are non-conductive, this rate changes to between 0.0059 birds/pole/year and 0.1298 birds/pole/year [1]. In addition to the large number of birds killed, electrocution more strongly affects female raptors compared to male raptors, which has a much stronger effect on the breeding capability of the species [16].

Problems in design

Different power pole designs have different rates of electrocution. This is due in part to how the wires are arranged and how easy it is for a raptor to span the distance between the two wires. The common North American 3 phase wooden distribution power pole is one of the most dangerous for raptors. This is caused by the propensity of the raptors to perch on the wooden crossbeam to rest or eat, when the raptor then goes to take off, the down sweep of the wings easily bridges between two conductors, and running the current through the bird. The mortality is increased when the power pole has a transformer, or bridges between two lengths of conductor. In areas with power poles made of materials other than wood, the mortality rate increases [1]. This issue occurs because steel and, to a lesser extent, concrete are conductive to electricity. What this means is that negative interactions between the raptors and power poles become more common. This happens because instead of the raptor needing to contact two conductors on a wooden pole, the raptor only needs to contact one live wire as the raptor will create a path to ground through the material of the pole, in a way that is not possible on wooden poles. [18]

Design solutions

A power distribution pole at a US naval facility fitted with multiple deterrents and safety devices. Mass Communication Specialist 1st Class R. Jason Brunson. Public domain, via US Navy

There are many solutions to help reduce raptor deaths on powerlines. One of the simplest, but also the least practical, is to put a minimum distance between conductors and between conductors and ground. This spacing recommended by the Avian Power Line Interaction Committee is 154.2cm horizontal spacing and 101.6cm vertical spacing [19]. On a simple residential 3 phase distribution power pole, this can create issues as the pole will span over 3m wide assuming three conductors abreast which can be problematic in crowded urban and suburban areas. Another solution is to install various perch deterrents in an effort to discourage the use of the power poles. While this solution is relatively easy to retrofit onto existing poles, some poles lack the weight capacity to install the deterrents. In addition, even when the deterrents are installed, raptors will still perch on poles, but may reduce the duration of the perching [19]. The easiest solution to the problem of raptor electrocution is to add or increase insulation on all wires on and around the towers. One issue with this method is that insulation degrades over time and so decreases in effectiveness. In addition to degradation, it requires human access to all towers to facilitate installation. A final solution is to install an insulated perch post or perch bar high enough above the conductors that a raptor cannot contact one or more conductors while taking off or landing. [20]

Nesting Habitat

Power line poles and pylons have been used in various parts of the world to aid conservation efforts of threatened raptor species. Sometimes artificial nests, nesting boxes, or platforms are placed on the poles, while in other cases the birds build their own nests using the power line infrastructure as a substrate. There are a number of success stories from the use of power lines, but whether their use as a nesting substrate should be tolerated or implemented as a management plan depends on the needs of the species and ecosystem under consideration. The following describes a number of case studies regarding the use of power lines in conservation efforts.

Martial Eagles

Most of sub-Saharan Africa’s eight large eagle species are threatened in at least part of their range. As apex predators large eagles naturally occur at low densities and reproduce slowly. They are thus particularly susceptible to anthropogenic disturbances and so it is important to utilize situations where human impacts on the environment can actually benefit eagles [3]. In the semi-arid Karoo region of South Africa the martial eagle, Verreaux’s eagle, and tawny eagle have all been documented using power lines as a nesting platform. Electrocutions on these large structures are rare and the eagles appear to benefiting from that region’s expanding power grid [3].

Martial eagle nest, South Africa. Martin Heigan. CC By-NC-ND 2.0, via Flickr

Ferruginous Hawks

Between 1980-1981 a 500-kv transmission line was installed through southern Idaho and Oregon [13]. From 1981-1989 nesting pairs increased from 3 to 133 along the line, particularly on the section parallel to the Snake River Canyon in southwest Idaho, with ravens being the most common of five bird species. Transmission towers provided both alternative and new nesting sites. For example, in 1982 pairs of breeding golden eagles shifted from existing natural nest sites to a tower 400 m away, while in 1983 a new breeding pair of golden eagles nested on a tower on the periphery of an occupied territory [13]. The combined density of nests in 1989 was 0.22 nests/km, or 8 nests per 100 towers [13]. This high density was presumably because natural nesting sites were lacking within 1-5 km of the line, while a population source existed less than 30 km away in the Snake River Canyon [13]. The increase of nesting birds along the transmission line was not at the expense of nesting birds in adjacent natural sites as the breeding populations of golden eagles, red-tailed hawks, ferruginous hawks, and common ravens remained as high or higher in the Snake River Canyon as before the time of this study [13]. Nesting success on towers was as high as or higher than for natural nesting sites. Towers provided protection from mammalian predators, especially in the case of ferruginous hawks which nest closer to the ground, protection from heat through increased wind and shade, and rangeland fire protection [13]. The biggest risk seems to have been nest destruction by wind. Platforms and other nest substrate enhancements added to towers would improve breeding success as even the dense steel latticework upon which nests were usually built failed to support large eagle nests in high winds [13]. The increase in nesting habitat and breeding population for the ferruginous hawk is particularly significant as they were thought to be declining and in 1980 were listed as threatened [21].

Saker Falcon

Placing artificial nests for Saker falcon on electric poles. Vladimirdobretic. CC BY-SA 3.0, via Wikimedia Commons

The IUCN lists the saker falcon as vulnerable [22], while the European population is endangered [22]. Due to various anthropogenic disturbances, breeding falcons in western Slovakia have moved from forested and mountain landscapes to lower elevation agricultural lands [22]. Because of the losses of natural nest habitats artificial nests were first installed on electrical pylons in 1994, and increased to over 150 by 2010 [22]. They were usually inhabited by the falcons within 1-2 years after installation and breeding success in them was higher than for natural nests or artificial ones placed in trees [22]. The higher breeding success is likely due to less human interference and increased availability of food [22]. Insulators were installed on pylons found within those agricultural areas and were expected to be on all pylons within breeding areas of western Slovakia by 2015 [22]. A similar trend of saker falcon movement has been seen in the Czech Republic and Hungary, where electrical pylons have also been used as a nesting substrate [22]. Since the introduction of artificial nests the saker population in this area has increased 145% and the nesting area has increased 3X. At this point the only way for breeding sakers to survive in Slovakia is through the use of these artificial nests [22]. A reason the artificial nests were so successful was likely because the saker falcon is amongst the most adaptable of all raptors in their use of nest material and substrates and in their choice of prey. Such management practices will need to be implemented for their entire European range if they are to survive there [22].

Saker falcon and power line, Hungary. Francesco Veronesi. CC BY-NC-SA 2.0, via Flickr

Buzzards

More broadly, many studies have shown greater breeding success in artificial nests than in natural nests, for eagles in Belarus, osprey in Belraus, Finland, and Canada, and ferruginous and Swainson's hawk in North America [23]. However, experiments with goshawk, common buzzard, and honey buzzard in Finland have revealed lower nesting success for artificial nests than for natural nests. Although nests were placed as far away from anthropogenic disturbances as possible, it is possible the birds were picking up on cues when choosing nest sites that are not apparent to humans. When artificial nests were used, brood sizes were the same as for natural nests - a sign of good site quality [23]. The lower success rate in the case of the goshawk and buzzards relative to the other studies can be explained by differences in the type of artificial nests that must be used: the former birds are smaller and use stick nests within or beneath the canopy, while the artificial nests in the other studies are for large raptors and were exposed on a pole or tree top, often in platform form [23].

Ecological Trap

There are concerns with using artificial nests for conservation purposes, as they may act as an “ecological trap”. In good quality territories birds may switch from natural nests to artificial ones that have a lower success rate, while in degraded sites where the best hope for survival would be to find a new territory, artificial nests may remove the motivation to leave [23].

Increased Predation

An increase in nesting habitat because of power line infrastructure can at times cause a raptor population to increase to the point of unsustainable predation of prey species. One example is a new transmission line in north-central Nevada where a concern was that perching raptors would predate endangered sage-grouse [4]. Researchers tested perching deterrents consisting of an inverted-Y exceeding the grip capability of large raptors [4]. Deterrents were found to lead to a decrease in raptor perching time and a lower probability of raptors being found in the area, but it did not stop perching behaviour completely. Deterrents can be a useful management tool, but a more economic approach to sage-grouse conservation would be to focus on habitat management [4].

Another example involves the pied crow, a generalist, widely distributed, pan-African bird with a reputation of causing damage to other raptor populations through mob harassment, competition, and nest predation [5]. Power line infrastructure in treeless areas, along with a warming climate, is thought to be responsible for an increase in their population numbers [5].

Layering Perspectives

In this section we invite contributions from scholars and students to widen the scope of the issues and possible solutions presented in this case study.

Conclusion

Power lines create a unique opportunity and challenge for the conservation of raptors. On the one hand, the tall poles create ideal nesting and perching sites for birds whose natural habitats are often in decline and can lead to an increase in their breeding population. On the other hand, electrocution is a leading cause of raptor mortality and decreasing population numbers. Conservation strategies for threatened species should simultaneously work to create nesting habitat on power lines, while mitigating potential dangers of electrocution. A mix of tools such as artificial platforms, perch deterrents, insulators, and wider spacing between electrified elements should be used. Conservation groups, various levels of government, and utility companies should work together and share the burden and costs of research and implementation. In cases where habitat crosses borders, as with the saker falcon in Slovakia and Hungary, or in the case of migrating birds, international agreements and standards should be created to ensure that power line structures help rather than hinder the cause of conservation. Monitoring of these lines and nests should be implemented to ensure that all devices are in good working order and that raptor populations are doing well, but also not increasing past the sustainable limits of their environment.

References

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