Do birds in flight respond to (ultra)violet lighting?

To satisfy the energy demand and to increase the share of renewable resources wind energy deployment has increased (Intergovernmental Panel on Climate Change 2011). Concerns about the impact of wind energy on birdlife, through collisions, disturbance and habitat loss, have at the same time become more acute (Gove et al. 2013). Because of the fast rate of deployment, it has become challenging to verify impacts on birdlife and develop ways of mitigating these (Gove et al. 2013). These concerns can be an economic problem for the energy industry and for society as a whole (Cole 2011), and reduces the predictability in the planning and consenting processes. Collision with wind turbines is the main cause for direct bird mortality at wind-power plants. However, mitigating wind-turbine induced bird mortality is particularly complicated because it may originate from collision, disturbance and barrier effects that are site- and species-specific (Marques et al. 2014; May et al. 2015). Concerns regarding collision risk have stimulated research to quantify these effects post-construction, and to predict the extent of effects pre-construction in connection with the planning of new wind-power plants. Developing and testing techniques for mitigating collision risk, however, still need to be improved (Intergovernmental Panel on Climate Change 2011). It is paramount to develop practical and functional tools, products and other measures that reduce bird mortality related to offshore and onshore wind energy production, in order to avoid delay in consenting processes and to streamline the construction and operation phase while preserving bird populations at those sites.

In practice, it is normally a very long step from documenting the extent of the impact caused by the construction and operation of wind-power plants to successful mitigation (Lehman et al. 2007). The main reason why mortality-reducing tools are not already developed is due to the challenges in assessing the effectiveness of such tools in situ (May et al. 2015). The detected number of birds killed at a wind-power plant is often too low, due to removal by scavengers or observer bias, to be used as the only criteria to assess a measure’s effectiveness within a limited timeframe. Studies of behavioural responses, such as avoidance and/or reduced activity in the vicinity of the turbines due to the measures tested, are therefore necessary (May et al. 2017). Due to the relatively short history of wind energy production, there is a lack of comparative studies of scientific quality (May et al. 2015). Investigations in this field are connected with several uncertainties such as statistical significance, time required to get reliable results, number of trials/turbines, costs and practicalities, especially in connection with offshore installations. Still, in situ testing of promising pathways to mitigate impacts are important to increase our knowledge incrementally. One promising approach is that of utilizing the sensitivity of many bird species within the (ultra)violet spectrum to deter birds from turbines using ultraviolet or violet (UV) lighting (May et al. 2015). Visual deterrence using UV lights will be most effective at low light levels, and may therefore mainly help to mitigate collisions of nocturnal and crepuscular flight activity between dusk and dawn. Still, to-day there exists no proof-of-concept whether birds are in fact deterred in flight from areas lit up by (ultra)violet light (but see Hunt and McClure 2015).

Vision is the primary sensory system of birds, with a higher acuity and a generally lower temporal resolution than humans. It will therefore be important to take into account avian visual ecology when designing mitigation measures (Martin 2011). Blackwell and Bernhardt (2004) and Blackwell et al. (2009, 2012) empirically showed that birds’ visual systems (visual acuity and visual fields) enable them to respond behaviourally to lighting regimes (no, constant or pulsating lights) in approaching objects (aircraft and vehicle). Birds have tetrachromatic colour vision and spectral sensitivities of photoreceptors between 320 and 700 nm (Osorio and Vorobyev 2008). Birds with highly sensitive UVS-cones and birds with the less UV-sensitive VS-cone variant are all sensitive to UV light (Lind et al. 2014). Both gulls and passerines are sensitive within the ultraviolet spectrum (355–380 nm: Ödeen et al. 2010; Lind et al. 2014). Gulls are species of concern for collision with offshore wind turbines given their ecology (Furness et al. 2013). Raptors and owls, including Burrowing Owl (Athene cunicularia) and Golden Eagle (Aquila chrysaetos), seabirds and waders, and gallinaceous birds have (ultra)violet single cones with sensitive functions that peak in the violet spectrum (402–426 nm: Håstad et al. 2005; Ödeen and Håstad 2013; Doyle et al. 2014; Lind et al. 2014). At onshore wind-power plants, raptors and owls are often perceived to be vulnerable to collision mortality (Drewitt and Langston 2008). Typically, the lower limit of the human visible spectrum is 390 nm. Given these differences, it may be possible to develop mitigation measures without disturbing humans. Here we have to keep in mind that objects reflecting or emitting ultraviolet light may, besides having an illumination effect, be viewed as a different colour to the avian eye (Cook et al. 2011). Although the avian eye is better than ours—in avoiding obstacles or chasing prey in fast flight—its performance decreases with falling light levels more so than for humans (Jarvis et al. 2002), perhaps even during rain or cloudy weather. The function of ultraviolet vision in birds is thought to be related to orientation, foraging and signalling (Bennett and Cuthill 1994). Ultraviolet cones in the eyes of birds have been shown to be receptive to both visual and magnetic information (Bischof et al. 2011). Although monochromatic ultraviolet light (373 nm) could disrupt natural orientation behaviour in European Robins (Erithacus rubecula) (Wiltschko et al. 2014); these two mechanisms (vision and magnetoreception) were found to be independent of each other. Birds may be able to separate visual and magnetic information derived from ultraviolet cones through light at other wavelengths, magnetite-based receptors in the beak, and/or optic flow during movement (Bischof et al. 2011; Wiltschko et al. 2014). In fact, Gauthreaux and Belser (2006) summarize how filtering out all wavelengths but for ultraviolet light essentially eliminated migratory bird mortality at ceilometers. Also, Poot et al. (2008) showed that nocturnally migrating birds were least disoriented from blue lights (455 nm).

Although it might be tempting to operationalize innovative mitigation tools solely based on theory, pre-development ex situ feasibility studies as well as post-development in situ testing are important prior to deployment. Such assessments ensure that unsuccessful techniques can be eliminated in an early phase and the effectiveness of innovations can be documented. Lights within the UV spectrum (≤ 400 nm) with low power input are now available. Martin (2011, 2012) argues that the sensory ecology in birds is more attuned to observing and responding to impulses from the ground. We therefore envision utilizing UV lights that sweep upwards during nighttime encircling the rotor swept zone to exclude birds from this risky area (i.e. light-fence). UV lights are invisible to the human eye but may deter nocturnal birds from entering the rotor swept zone without creating visual nuisance for humans. However, there are potential health hazards connected with UV light, which can be harmful to peoples’ (and birds’) eyesight that will have to be taken into account. Such a system must therefore be constructed in such a way that it will be safe to the public, which means that it must be mounted out of reach of people (well above the base), with the light beam pointing upwards, perhaps in the form of pulsating beams. However, before commencing with intricate engineering to conceive optical deterring devices to birds, it will be important to know beforehand whether bird species that are sensitive within the (ultra)violet spectrum actually respond behaviourally to UV light. As a feasibility study, we therefore explored whether birds in flight respond to ultraviolet/violet lights. In theory, UV lights may either lead to behavioural evasion of the birds in flight (partial avoidance, with a shift in their flight path) or displacement from the lighted area (leading to reduced activity) (May 2015). Alternatively, birds might also be attracted to the UV lights leading to opposite results. The main hypothesis for testing the possible efficacy of UV lights was that birds would refrain from flying through the area of light. This hypothesis leads to the following research questions: (1) to which extent UV light leads to a proportional reduction in the number of recorded flights within pre-specified distances and altitudes around the lighted area, and (2) to which extent recorded flights verifiably change altitude, and at which distance? These effects may in addition change over time due to positive (getting used to the lights) or negative (avoiding the area altogether) habituation.

This pilot study indicated that UV-light displaced birds in the vicinity of the light source. While birds were partially displaced in altitude by the emitted light at 365 nm, birds mostly adjusted their flight altitude when subjected to light of 400 nm. This displacement effect persisted over the season below 20–30 m a.s.l. but with increasing use of higher altitudes over time. This may indicate habituation over time, but could also just occur at the periphery of the lit area. Verification of this would require long-term studies and could not be determined in the current study. The test design consisted of UV-lights placed 2.5 m off the ground and pointing upwards; observed displacement is therefore also relative to ground level. However, when implemented, the UV-lights at a wind turbine have to be placed such that they encompass the rotor-swept zone; i.e. 50–100 m above ground level. To which extent displacement will occur at higher altitudes above ground level could not be answered by this study. Also, in order to be effective in deterring birds from the rotor swept zone of a wind turbine, most birds need to display avoidance along the entire rotor blade length (40–50 m). This study indicated that, given the lights used, only a proportion of birds avoided the lights to up to 30 m at a maximum. However, when the light is allowed to reflect off the surface of the turbine blades, e.g. using a UV-reflective coating (Young et al. 2003), it is possible that this would extend the range of the deterring effect, compared to this experiment that used an isolated lamp.

To increase the efficacy of the UV lights to deter birds from the area, this necessitates that the energy emitted from the lights are diffused as little as possible, using either higher output or a beam that is as narrow as possible. However, with increased irradiance (Watt/m²) and/or narrower beam width, also the minimum safety distances increase within which eye damage may occur. The efficacy of UV light as a deterrence measure likely also depends on the solar irradiance levels to reach above-solar threshold values. The time-period when lights emit more UV-light than the sun, diminishes towards midday and varies seasonally. This likely also varies at different latitudes with higher solar irradiance towards the equator but more variable daytime periods at higher latitudes.

Lastly, in practical applications, potentially harmful ecological effects of UV light pollution should be considered, such as attraction of insects (van Langevelde et al. 2011). Attraction of insects may in turn attract birds, and especially bats, to the wind turbines resulting in increased collision risk in these species. Based on this pilot study, we recommend that further research is required investigating physiological vision damage distance thresholds in birds and humans (dependent on how the lights will be directed), testing behavioural responses in birds to other lighting regimes (e.g. narrower beam width or laser, moving or flickering beam) in combination with potential side effects (e.g. insect attraction, light pollution). This pilot study has shown that birds were partially displaced by UV-light within a limited distance range, which may allow the development of mitigation designs only encompassing the rotor swept zone potentially combined with a triggering system to minimize habituation (May et al. 2015). This may in turn, when tested successfully in situ, help reduce collision risk while maintaining utilization of the area in-between the wind turbines by birds.

Vision is the primary sensory system in birds, which for a number of species also includes the ultraviolet spectrum. Many bird species that are known to collide with offshore wind turbines are sensitive in the violet or ultraviolet spectrum. For species that are mainly active at lower ambient light levels, UV lighting may deter birds from the lit area. However, we do not know whether UV-sensitive birds in flight actually respond behaviorally to UV lights. In this study we found that, relative to control nights, bird flight activity was reduced within the lit area when the (ultra)violet light was on. In addition, the avian radar based data showed a limited vertical displacement in flight altitude which persisted over the season below 40 m above sea level. Still, with regard to implementation, we argue there still is a long way to go before a potentially functional design to mitigate collisions at offshore wind turbines that has proven to be effective in situ may be in place.