Mapping the relative risk of impact on nature by a human infrastructure at a landscape scale (“sensitivity mapping”) is an essential tool for minimising the future impact of new development or for prioritising mitigation of existing impacts. High-voltage power lines (“transmission lines”) are known to increase bird mortality by collision. Here we present a method to derive a high resolution map of relative risk of transmission line impacts across one entire country, Belgium, from existing bird distribution data. First, all the bird species observed in Belgium were systematically assessed using literature and casualty records to select those to be included in the sensitivity map. Species were selected on the basis of their intrinsic susceptibility to collision and the conservation relevance of avoiding additional mortality for that species in Belgium. Each of the selected species was included in one or several spatial layer constructed from existing data, emerging from citizen science bird monitoring schemes. The resulting 17 layers were then combined into one final sensitivity map, where a “risk score” estimates the relative collision risk across Belgium at a 1×1 km resolution. This risk score is relatively robust to the subtraction of any of the 17 layers. The map identifies areas where building new transmission lines would create high risk of collision and, if overlapped with existing power lines, helps to prioritise spans where mitigation measures should be placed. Wetlands and river valleys stand out as the most potentially dangerous areas for collision with transmission lines. This sensitivity map could be regularly updated with new bird data or adapted to other countries where similar bird data are available.

Belgium, bird, mitigation, sensitivity mapping, strategic planning, transmission lines, waterbird

Power lines have been identified as one of the major causes of man-induced mortality in birds (Loss et al. 2015). Direct mortality by collision with overhead wires is known to occur with any type of lines but has been especially studied for the so-called transmission grid, or high-voltage power lines (Bernardino et al. 2018), while medium-voltage lines (the so-called distribution grid, from 1 to 30 kV in Belgium) additionally induce electrocution risk for larger birds (Guil and Pérez-García 2022). Flying birds might collide with wires especially under low visibility conditions during crepuscular or nocturnal movements or during bad weather. Despite the difficulties inherent to such quantification, casualty numbers are undoubtedly very high. An annual estimate of 8–57 million birds killed by collision with transmission lines was made for the USA alone (Loss et al. 2014). Despite these impressive figures, only in very few instances has a link been established between population dynamics (e.g. the decline of a given population of a single bird species) and power line mortality (Bernardino et al. 2018; D’Amico et al. 2018). Possible demographic impacts mostly concern endangered species: 12% of the entire Blue Crane Anthropoides paradiseus population was estimated to be killed annually by collision with power lines in South Africa (Shaw et al. 2010). On the other hand, low mortality rates do not mean that no significant impact exists, e.g. by historical depletion of local populations (Ascensão et al. 2019). Even if a population effect of power line mortality cannot be readily established, it is important to reduce this human-induced mortality as much as possible in an attempt to minimise the impact of this ever-growing infrastructure. An estimated 65 million kilometres of medium- to high-voltage lines was already covering the world a decade ago (Jenkins et al. 2010). With the anticipated change towards a more decentralised production, transmission grids are expected to grow constantly in the near future (Barov 2011; Bio Intelligence Service 2012; Biasotto and Kindel 2018). Therefore, minimising bird fatalities on both existing and future power lines is critical and a prerequisite to increase public support for such a development.

Construction of underground lines is the best solution to prevent any further casualties. However, this is not always possible from a technical point of view or economically viable, especially when existing aerial lines have to be brought underground. Wire marking in order to increase visibility of the cables for birds is therefore the most widespread measure to reduce mortality. A recent review of wire-marking effectiveness (Bernardino et al. 2019) concluded that mortality rates on average are reduced by half (95% confidence interval: 40.4–58.8% across 35 studies).

Strategic planning has been proposed as a first necessary step to mitigate power line impact, both to avoid building new power lines in vulnerable areas and to act on mitigation measures on existing dangerous lines (Bernardino et al. 2018; D’Amico et al. 2018). Sensitivity mapping is routinely used in several instances of interaction of fauna with human infrastructure as a basic planning tool when new infrastructure has to be built (Allinson et al. 2020). It also helps to prioritise where mitigation measures should be taken (European Commission 2018). However, there are very few examples of sensitivity maps for a countrywide transmission line network (but see (D’Amico et al. 2019) for Portugal and Spain). Here, we present a method based on large-scale citizen science data to map the relative collision risk associated with transmission lines for birds, for any given location in Belgium. We also propose a prioritisation process to mitigate the risks associated within the existing 5,614 km of aerial transmission lines (70–380 kV) in Belgium.

Identifying existing transmission lines presenting a high collision risk for birds or drawing attention to potentially harmful future lines can be attempted at a regional scale by looking at some natural habitat features or spatial characteristic (Martín Martín et al. 2019), but mapping areas where collision-susceptible species are particularly abundant would always give a better assessment of the risk, especially on a larger scale. Indeed, not all species are facing the same risk when confronted with power lines since some species are more prone to collisions than others (Bevanger 1998). These susceptible species could congregate in large numbers at specific places used on a daily basis, like communal night roosts or breeding colonies, hence increasing the number of potential casualties around those areas. Besides the intrinsic probability of collision for a given species, the conservation value (for example, their IUCN status) or demographic sensitivity to higher adult mortality could also guide the choice of lines to be targeted for mitigation.

The building of the collision risk map in Belgium followed several steps. First, a list of bird species prone to collision with power lines has been compiled based on a review of the literature and casualty records in Belgium. This list was then matched with available recent data on bird distribution and abundance, provided by different schemes of large-scale bird monitoring and a citizen-science portal. Several layers of spatial information on birds were then combined using a scoring system to create a sensitivity map at a resolution of 1×1 km. When overlapped with the existing transmission line network, this map highlights power line spans presenting high collision risk for birds and is now used by the transmission system operator in Belgium, Elia, to define priority sectors for mortality surveys and, more importantly, mitigation actions. Furthermore, the risk map allows for the planning of new developments of the transmission grid minimising collision risk, but not precluding the necessity of environmental impact assessment to detect possible collision issues before any new line construction.

The list of susceptible species to be considered for collision risk with transmission lines amounts to 83 bird species in Belgium (see Suppl. material 1: Table S1 for the complete list). This represents 38.4% of all regularly observed bird species in Belgium. Thanks to regular coordinated monitoring of wintering waterbirds, colonial breeding birds and some socially roosting species, together with a very popular bird recording system (about 2 million bird records in Belgium every year), a large number of data could be used to draw the 17 thematic layers (all presented as Suppl. material 4: Figs S1–S17).

The application of the scoring system resulted in a map at 1×1 km spatial resolution for collision risk with power lines for Belgium, presented in Fig. 1. This map is independent of the presence of actual power lines; it represents a hypothetical risk based on the additive presence of the identified sensitive species.

Figure 1. 

The transmission lines collision risk map for the whole of Belgium, shows the risk at any location in the country. This is a theoretical score not accounting for the current presence/absence of a power line, based only on the additive presence or high abundance of the sensitive species.

Combining all the possible maximum scores for each layer, the theoretical highest possible score is 176. In our present assessment, the highest observed score is 153. There is a clear gradient of risk from the lowlands in northern Belgium, where most wetlands are located, to southern, more elevated parts of the country, where risk is more diffused except along the main river valleys. The polder areas are the most critical areas as these are major concentration sites for waterbirds. Inland wetlands are also focal points for collision risk.

When overlapped with the risk maps, power-line spans (the linear segment of lines between two pylons) can be classified according to the relative risk they represent to birds (Fig. 2). The most dangerous span in the present assessment is predicted to be the line crossing a nature reserve along a major tributary of the Scheldt river, with a score of 133.

Figure 2. 

Map of the existing transmissions lines, colour-coded according to the bird collision risk they represent. Most of the high-priority lines are close to important waterbird sites, but numerous segments are also located in the central part of the country, in the historically industrial river valleys.

Most of the lines run through medium- or low-risk score areas (Fig. 3). Looking at the grid as a whole, 5.8% of the total length has a score above 80.

Figure 3. 

Frequency distribution of grouped risk scores for the total length of overhead line spans (for the whole of Belgium).

Depending on which data layer was removed, 81.6% – 90.1% of the most dangerous grid cells (as identified by the complete risk map) remained within the top 10% of the most dangerous grid cells (according to the reduced risk maps, Suppl. material 3: Table S4). This indicates a certain robustness from the collision risk map to the removal of one specific data layer.

Reducing the risk of bird mortality along transmission lines is an important goal to achieve in a context where electricity transport system will inevitably expand throughout the world. Here we propose a method based on existing bird data to identify the “dark spot” where collision risk is relatively higher at a country scale, the scale at which the transmission line companies are operating. We believe that such an approach could inform the strategic planning of new transmission lines to be installed but more directly could be used to target mitigation actions – wire marking – on existing lines, once the existing network is overlapped with our risk map. A similar sensitivity mapping approach was developed previously in Spain and Portugal, taking into account susceptible breeding bird distribution at the scale of 10×10 km (D’Amico et al. 2019). Here, both breeding and wintering bird abundances were brought into the map at a resolution of 1×1 km, thanks to the spatially explicit data provided by several citizen-science schemes.

Our results indicate that the risk of bird collisions with high-voltage power lines is unequally distributed over Belgium. This knowledge is important for multiple reasons. Firstly, for existing power lines, it contributes to focusing efforts to mitigate effects as efficiently as possible, where every investment has the highest return translated into prevented collision casualties. Secondly, the country wide risk assessment (independent of the presence of a transmission line) can be used to compare potential trajectories of new proposed power lines.

The collision risk map was entirely based on data about the avifauna. However, the risk of bird collision is not only depending on the species richness and the abundance of birds, but also on the technical configuration of the pylons and consequently the power lines. Spacers, which separate the lines of the phase, can increase visibility (Bevanger 1994). The height of the power line is also likely to affect the bird collision risk, as is the number of vertical wire levels, the wire diameter and the presence of an earth wire (Bernardino et al. 2018). Although currently not available nationwide (Mortier, J. pers. comm.), the addition of a technical data layer to combine with the risk derived from the avifauna data could refine the current results. Furthermore, there is the possible effect of the surrounding landscape. A power line located in a heavily forested habitat with power pylon height lower than the average tree height poses limited risk to possibly susceptible species since they are forced to fly above the trees and the power lines (Jenkins et al. 2010). We suggest taking these landscape elements into consideration for fine-tuning of the wire marking once mitigation has been targeted with the help of the countrywide risk map. However, even with a further refinement of this theoretical approach, it should not replace a detailed field survey of mortality along existing lines or the necessary field expertise necessary for a proper Environmental Impact Assessment.

A key issue in this sensitivity mapping approach is the availability of bird data at a country-wide scale. Our study area, Belgium, benefits from a high density of amateur birdwatchers and long-term coordinated monitoring schemes. But we think that our approach could be used even in less surveyed regions. Spatial modelling techniques are now available to produce reliable predictive spatial models based on citizen-science records, taking into account strong spatial bias in their collections (Tang et al. 2021). These citizen-science records are now starting to accumulate almost everywhere in the world and are generally available as open source data (Callaghan and Gawlik 2015; de Vries and Lemmens 2021). In our case, for species with a low detection rate, as Eurasian Woodcock, we could use the limited number of available data to estimate the total range at 1×1 km resolution. Scarcity of data should not prevent attempting to perform a risk map analysis in other regions of the world as we have shown that prioritised segments are rather constantly highlighted by the risk maps, even when removing one layer.

A common problem with many conservation assessments published is that they often do not result in any conservation action (Knight et al. 2008; Arlettaz et al. 2010; Schuwirth et al. 2019). Our sensitivity mapping was commissioned by Elia, the transmission lines operator in Belgium. An earlier version of the risk map (Derouaux et al. 2012) was already used by the company to prioritise mitigation actions and to equip with wire marking around 115 km of lines until 2021 across Belgium (around 2% of all lines; data Elia). Some of this wire marking already took place before the production of the first version of the risk map, but already 7.4% of the transmission lines with a risk score higher than 80 are now equipped with wire marking (Elia data). In several of these spans, before-after control impact treatment involving field searches of bird casualties are now under way. Future field work analyses will allow for an assessment of the effectiveness of the prioritised wire markings but also will provide an evaluation of the theoretical mapping approach presented here.

Once established, our risk map analysis could be easily updated with new data, as bird monitoring and data collecting programs involved are running continuously and bird numbers and distributions are often susceptible to rapid changes. Another potential use of our risk analysis method is to assess further needs in wire marking (or burying) in the case of major natural wetlands restoration programmes (Decleer et al. 2016) that could result in large-scale bird distribution changes (Bregnballe et al. 2014) and thus changing the collision risk associated with existing transmission lines.

This work was only made possible thanks to the dedication of thousands of enthusiast field observers and ornithologists. The value of their mostly voluntary field work is further enhanced by the expert validators, census organisers and database curators of Natuurpunt, Natagora, Département d’Études des Milieux Naturels et Agricoles from the Service Public de Wallonie, the Research Institute of Nature and Forest in Flanders. This work was commissioned by Elia. The authors specially wish to thank Johan Mortier (Elia) for his continuous support. An earlier version of this manuscript was greatly improved thanks to the suggestions of two anonymous referees.