Phenology of Migratory Bat Activity Across the Baltic Sea and the South-Eastern North Sea

Toward solving the global green-green dilemma between wind energy production and bat conservation

Wind energy production is growing rapidly worldwide in an effort to reduce greenhouse gas emissions. However, wind energy production is not environmentally neutral. Negative impacts on volant animals, such as bats, include fatalities at turbines and habitat loss due to land-use change and displacement. Siting turbines away from ecologically sensitive areas and implementing measures to reduce fatalities are critical to protecting bat populations. Restricting turbine operations during periods of high bat activity is the most effective form of mitigation currently available to reduce fatalities. Compensating for habitat loss and offsetting mortality are not often practiced, because meaningful offsets are lacking. Legal frameworks to prevent or mitigate the negative impacts of wind energy on bats are absent in most countries, especially in emerging markets. Therefore, governments and lending institutions are key in reconciling wind energy production with biodiversity goals by requiring sufficient environmental standards for wind energy projects.

Acoustic monitoring reveals spatiotemporal occurrence of Nathusius' pipistrelle at the southern North Sea during autumn migration

Seasonal movements between the summer and winter areas are a widespread phenomenon in bats So far, most information on the migration ecology of bats has been obtained by studies in terrestrial habitats, whereas scientific knowledge on migration over sea is scarce. We performed continuous ultrasonic acoustic monitoring at 13 locations in the southern North Sea during four consecutive years (2017-2020) and analysed the spatiotemporal occurrence of Nathusius' pipistrelle Pipistrellus nathusii during autumn migration in relation to weather parameters and lunar phase. Our analysis showed that the main autumn migration of Nathusius' pipistrelle at the southern North Sea occurs from mid-August until late October and most bats within the study area occur off the Noord Holland coast. North Sea crossings frequently last longer than one night; the day is spent roosting at an offshore structure. The strongest migration occurs during nights with tailwinds from the east-northeast, but bats are also recorded offshore with low to moderate headwinds or crosswinds. Bat presence decreased between the full moon and the last quarter and increased just before the new moon. Finally, our observations show that the occurrence of bats at sea was reduced in 2020 in comparison to the previous years. The results of this study show clear spatiotemporal patterns of migratory bat occurrence at the southern North Sea. The spatial distribution can be used in spatial planning of future offshore wind farms, whereas the temporal occurrence and environmental factors that shape offshore migration can be used to develop mitigation measures to reduce the number of bat fatalities.

Foraging and mating behaviors of Hypsignathus monstrosus at the bat-human interface in a central African rainforest

Studying wildlife space use in human-modified environments contributes to characterize wildlife-human interactions to assess potential risks of zoonotic-pathogens transmission, and to pinpoint conservation issues. In central African rainforests with human dwelling and activities, we conducted a telemetry study on a group of males of Hypsignathus monstrosus, a lek-mating fruit bat identified as a potential maintenance host for Ebola virus. During a lekking season in 2020, we investigated the foraging-habitat selection and the individual nighttime space use during both mating and foraging activities close to villages and their surrounding agricultural landscape. At night, marked individuals strongly selected agricultural lands and more generally areas near watercourses to forage, where they spent more time compared to forest ones. Furthermore, the probability and duration of the presence of bats in the lek during nighttime decreased with the distance to their roost site but remained relatively high within a 10 km radius. Individuals adjusted foraging behaviors according to mating activity by reducing both the overall time spent in foraging areas and the number of forest areas used to forage when they spent more time in the lek. Finally, the probability of a bat revisiting a foraging area in the following 48 hours increased with the previous time spent in that foraging area. These behaviors occurring close to or in human-modified habitats can trigger direct and indirect bat-human contacts, which could thus facilitate pathogen transmission such as Ebola virus.

Coastal onshore wind turbines lead to habitat loss for bats in Northern Germany

Wind energy production is particularly rewarding along coastlines, yet coastlines are often important as migratory corridors for wildlife. This creates a conflict between energy production from renewable sources and conservation goals, which needs to be considered during environmental planning. To shed light on the spatial interactions of a high collision risk bat species with coastal wind turbines (WT), we analysed 32 tracks of 11 common noctule bats (Nyctalus noctula) in Northern Germany with miniaturized global positioning system units yielding 6266 locations. We used three spatial models to infer on the preferred and avoided landscape features in interaction with WT. We found 3.4% of all locations close to WT, with bats preferring areas with high levels of impervious surface, identified as farmhouses. Common noctule bats were also more present close to WT adjacent to paths and waterbodies. At the local scale, >70% of common noctule bats avoided WT, yet if bats approached WT we counted more positions at large WT, specifically close to known roosts. Our study highlights that coastal WT should not be placed next to feeding grounds and bat roosts. Additionally, avoidance of WT by bats indicates that foraging bats may suffer from habitat loss in coastal landscapes with high turbine densities. To mitigate the conflict between wind energy power production and conservation goals at coastal sites, wind turbines should be placed at distance to habitat features preferred by bats and turbine densities should be limited.

The Relation between Migratory Activity of Pipistrellus Bats at Sea and Weather Conditions Offers Possibilities to Reduce Offshore Wind Farm Effects

Bats undertaking seasonal migration between summer roosts and wintering areas can cross large areas of open sea. Given the known impact of onshore wind turbines on bats, concerns were raised on whether offshore wind farms pose risks to bats. Better comprehension of the phenology and weather conditions of offshore bat migration are considered as research priorities for bat conservation and provide a scientific basis for mitigating the impact of offshore wind turbines on bats. This study investigated the weather conditions linked to the migratory activity of Pipistrellus bats at multiple near- and offshore locations in the Belgian part of the North Sea. We found a positive relationship between migratory activity and ambient temperature and atmospheric pressure and a negative relationship with wind speed. The activity was highest with a wind direction between NE and SE, which may favor offshore migration towards the UK. Further, we found a clear negative relationship between the number of detections and the distance from the coast. At the nearshore survey location, the number of detections was up to 24 times higher compared to the offshore locations. Our results can support mitigation strategies to reduce offshore wind farm effects on bats and offer guidance in the siting process of new offshore wind farms.

Identifying migratory pathways of Nathusius' pipistrelles (Pipistrellus nathusii) using stable hydrogen and strontium isotopes

RATIONALE

Identifying migratory corridors of animals is essential for their effective protection, yet the exact location of such corridors is often unknown, particularly for elusive animals such as bats. While migrating along the German coastline, Nathusius' pipistrelles (Pipistrellus nathusii) are regularly killed at wind turbines. Therefore, we explored the paths taken on their annual journey.

METHODS

We used isotope ratio mass spectrometry to measure stable hydrogen and strontium isotope ratios in fur keratin of 59 Nathusius' pipistrelles captured on three offshore islands. Samples were pre-treated before analysis to report exclusively stable isotope ratios of non-exchangeable hydrogen. We generated maps to predict summer origins of bats using isoscape models.

RESULTS

Bats were classified as long-distance migrants, mostly originating from Eastern Europe. Hydrogen analysis suggested for some bats a possible Fennoscandian origin, yet additional information from strontium analysis excluded this possibility. Instead, our data suggest that most Nathusius' pipistrelles migrating along the German coastline were of continental European summer origin, but also highlight the possibility that Nathusius' pipistrelles of Baltorussian origin may travel offshore from Fennoscandia to Germany.

CONCLUSIONS

Our findings demonstrate the benefit of using complementary isotopic tracers for analysing the migratory pathways of bats and also potentially other terrestrial vertebrate species. Furthermore, data from our study suggest an offset of fur strontium isotope ratios in relation to local bedrock.

Climatic drivers of (changes in) bat migration phenology at Bracken Cave (USA)

Climate change is drastically changing the timing of biological events across the globe. Changes in the phenology of seasonal migrations between the breeding and wintering grounds have been observed across biological taxa, including birds, mammals, and insects. For birds, strong links have been shown between changes in migration phenology and changes in weather conditions at the wintering, stopover, and breeding areas. For other animal taxa, the current understanding of, and evidence for, climate (change) influences on migration still remains rather limited, mainly due to the lack of long-term phenology datasets. Bracken Cave in Texas (USA) holds one of the largest bat colonies of the world. Using weather radar data, a unique 23-year (1995-2017) long time series was recently produced of the spring and autumn migration phenology of Brazilian free-tailed bats (Tadarida brasiliensis) at Bracken Cave. Here, we analyse these migration phenology time series in combination with gridded temperature, precipitation, and wind data across Mexico and southern USA, to identify the climatic drivers of (changes in) bat migration phenology. Perhaps surprisingly, our extensive spatiotemporal search did not find temperature to influence either spring or autumn migration. Instead, spring migration phenology seems to be predominantly driven by wind conditions at likely wintering or spring stopover areas during the migration period. Autumn migration phenology, on the other hand, seems to be dominated by precipitation to the east and north-east of Bracken Cave. Long-term changes towards more frequent migration and favourable wind conditions have, furthermore, allowed spring migration to occur 16 days earlier. Our results illustrate how some of the remaining knowledge gaps on the influence of climate (change) on bat migration and abundance can be addressed using weather radar analyses.

Bats and Wind Farms: The Role and Importance of the Baltic Sea Countries in the European Context of Power Transition and Biodiversity Conservation

Although labeled as environmentally friendly, wind power can have negative impacts on the environment, such as habitat destruction or wildlife fatalities. Considering the distribution and migratory characteristics of European bats, the negative effects of wind power should be addressed on an appropriate scale. This review summarizes the current state of knowledge on interactions between wind farms and bats in Europe, and compares it with the situation in the countries of the European boreal biogeographic region. We analyzed data from papers published in international and national scientific journals, focusing on studies conducted in Europe. The issue of the impacts wind power has on bats is clearly overlooked in most of the countries of the European boreal region, with low volumes of research available on the topic. This is probably due to fewer wind farms in the area, making this recent issue a less-prioritized topic. However, the Baltic Sea, and the countries surrounding it, are of extreme importance with regards to bat migration, especially for the Pipistrellus nathusii. Therefore, more research on wind power and bats is needed in this region, as well as more cooperation between all the stakeholders.

First record of a Nathusius’ pipistrelle (Pipistrellus nathusii) overwintering at a latitude above 60°N

Highly mobile species are considered to be the first to respond to climate change by transforming their ranges of distribution. There is evidence suggesting that Pipistrellus nathusii, a species capable of long-distance migration, is expanding both its reproduction and overwintering ranges to the North. We recorded the echolocation calls of bats at 16 sites in South-Western Finland on two consecutive winters, and detected calls of P. nathusii at one of the sites throughout the second winter. To our knowledge, this is the northernmost record of an overwintering P. nathusii, and contributes to evidence that the species is already responding to climate change.

Assessing the use of rivers as migratory corridors for temperate bats

Investigating landscape-level movement patterns of migratory animals can be challenging, but this is a major component of some animal’s life history and behavior. In particular, bat migration has been difficult to characterize, yet recent research on bat migratory ecology has made major advances. It has been largely accepted that rivers and other linear landscape features may be important migratory corridors for bats during both long- and short-distance migrations. We assessed the migratory behavior of multiple temperate bat species along the Missouri River, a major river corridor in North Dakota, during March through October of 2016–2017. Bat detectors with paired microphones were deployed and oriented parallel to the riverbank. This configuration permitted detection of directional passes of bats, approximately 10–20 m above the microphones and 40 m into the river, which were used as an estimate of migratory behavior. We found the effects of season and species explained less than 2% of the variation of directional passes, indicating an absence of season-specific movement patterns along the studied river corridor. Although our study only assessed a portion of a major river corridor, the results suggest that migratory movements of bats along rivers may not be as straightforward as once thought, highlighting the need for future studies investigating the fine-scaled movement patterns of bats during migration.

Evidence for spring stopover refuelling in migrating silver-haired bats (Lasionycteris noctivagans)

Migrating animals must acquire sufficient fuel to sustain migratory movement, but how time is allocated to achieve this can vary greatly. The fuel strategies used by migrating bats are not well understood and have not been investigated during the spring when insectivorous bats face low food abundance. Migrating silver-haired bats (Lasionycteris noctivagans (Le Conte, 1831)) were captured at a stopover site in Long Point, Ontario, Canada, in April and May of 2012–2014. We followed the movements of 40 bats outfitted with radio transmitters using an automated telemetry array and examined the effects of ambient temperature, fat stores, and sex on stopover duration. As seen previously in autumn, most bats departed the evening following capture, but one-third of bats used multiday stopovers. Extended stopover was associated with lower ambient temperature. There was no effect of sex or fat at capture on stopover departure probability. Bats captured closer to dawn had greater fat mass and lean mass than those captured early in the night, a trend indicative of fuel deposition at this site. This is the first study to provide evidence that bats use stopover habitat for refuelling.

Experienced Migratory Bats Integrate the Sun's Position at Dusk for Navigation at Night

From bats to whales, millions of mammals migrate every year. However, their navigation capacity for accomplishing long-distance movements remains remarkably understudied and lags behind by five decades compared to other animals [1, 2]-partly because, unlike for other taxa, such as birds and sea turtles, no small-scale orientation assay has so far been developed. Yet recently, bats became a model to investigate which cues mammals use for long-range navigation, and, surprisingly for nocturnal animals, sunset cues, and particularly polarized-light cues, appear to be crucial for calibration of the magnetic-compass system in non-migratory bats [3-5]. This does not appear to hold for a species of migratory bat, however [6], and thus the nature of the information used by migratory bats for navigation remains unclear. Here, we asked whether the position of the solar disk per se is relevant for compass orientation in a migratory bat, Pipistrellus pygmaeus. Using a new experimental assay that measures takeoff orientation, we tested the orientation of bats exposed to a shifted sunset azimuth using a mirror at dusk. Bats exposed to a 180°-rotated azimuth of the setting sun and released after translocation during the same night shifted their heading direction by ∼180° compared to control bats. However, first-year migrants had no clear orientation either as controls or after the same treatment. This suggests that learning the migratory direction is a key component in the navigational system of naive bats in this species. Our study provides rare evidence for the specific cues and mechanisms that migratory mammals use for navigation.

Bat Migration☆

This article summarizes current knowledge about the migratory behavior of temperate and tropical bats. A close association between migration and hibernation exists in temperate, but not in tropical, bats. Compared with birds, bats are relatively short-distance migrators, with maximum migration distances being <2000 km; intercontinental migration is likely to be uncommon in bats. Migratory bats have lower levels of population subdivision and larger genetically effective population sizes than nonmigratory bats. A variety of methods, including banding, genetic analyses, stable isotope analyses, and tracking with radio or satellite transmitters, are currently being used to study bat migration. The conservation of migratory bats poses special challenges that require national and international efforts. Migratory bats sometimes harbor pathogens that can be transmitted to humans. Overall, the beneficial aspects of migratory bats, including control of insect populations and dispersal of pollen and seeds over broad areas, far outweigh their negative aspects.

Migratory bats are attracted by red light but not by warm-white light: Implications for the protection of nocturnal migrants

The replacement of conventional lighting with energy-saving light emitting diodes (LED) is a worldwide trend, yet its consequences for animals and ecosystems are poorly understood. Strictly nocturnal animals such as bats are particularly sensitive to artificial light at night (ALAN). Past studies have shown that bats, in general, respond to ALAN according to the emitted light color and that migratory bats, in particular, exhibit phototaxis in response to green light. As red and white light is frequently used in outdoor lighting, we asked how migratory bats respond to these wavelength spectra. At a major migration corridor, we recorded the presence of migrating bats based on ultrasonic recorders during 10-min light-on/light-off intervals to red or warm-white LED, interspersed with dark controls. When the red LED was switched on, we observed an increase in flight activity for Pipistrellus pygmaeus and a trend for a higher activity for Pipistrellus nathusii. As the higher flight activity of bats was not associated with increased feeding, we rule out the possibility that bats foraged at the red LED light. Instead, bats may have flown toward the red LED light source. When exposed to warm-white LED, general flight activity at the light source did not increase, yet we observed an increased foraging activity directly at the light source compared to the dark control. Our findings highlight a response of migratory bats toward LED light that was dependent on light color. The most parsimonious explanation for the response to red LED is phototaxis and for the response to warm-white LED foraging. Our findings call for caution in the application of red aviation lighting, particularly at wind turbines, as this light color might attract bats, leading eventually to an increased collision risk of migratory bats at wind turbines.

Migratory bats respond to artificial green light with positive phototaxis

Artificial light at night is spreading worldwide at unprecedented rates, exposing strictly nocturnal animals such as bats to a novel anthropogenic stressor. Previous studies about the effect of artificial light on bats focused almost exclusively on non-migratory species, yet migratory animals such as birds are known to be largely affected by light pollution. Thus, we conducted a field experiment to evaluate if bat migration is affected by artificial light at night. In late summer, we presented artificial green light of 520 nm wavelength to bats that were migrating south along the shoreline of the Baltic Sea. Using a light on-off treatment, we observed that the activity of Pipistrellus nathusii and P. pygmaeus, the two most abundant migratory species at our site, increased by more than 50% in the light-on compared to the light-off treatment. We observed an increased number of feeding buzzes during the light-on compared to the light-off treatment for P. nathusii. However, feeding activity was low in general and did not increase disproportionately during the light-on treatment in relation to the overall echolocation call activity of bats. Further, P. nathusii were attracted towards the green light at a distance of about 23 m, which is way beyond the echolocation detection range for insects of Nathusius’ bats. We therefore infer that migratory bats were not attracted to artificial green light because of high insect densities, but instead by positive phototaxis. We conclude that artificial light at night may potentially impact bat migration in a yet unrecognized way.

Habitat use of bats in relation to wind turbines revealed by GPS tracking

Worldwide, many countries aim at countering global climate change by promoting renewable energy. Yet, recent studies highlight that so-called green energy, such as wind energy, may come at environmental costs, for example when wind turbines kill birds and bats. Using miniaturized GPS loggers, we studied how an open-space foraging bat with high collision risk with wind turbines, the common noctule Nyctalus noctula (Schreber, 1774), interacts with wind turbines. We compared actual flight trajectories to correlated random walks to identify habitat variables explaining the movements of bats. Both sexes preferred wetlands but used conventionally managed cropland less than expected based on availability. During midsummer, females traversed the land on relatively long flight paths and repeatedly came close to wind turbines. Their flight heights above ground suggested a high risk of colliding with wind turbines. In contrast, males recorded in early summer commuted straight between roosts and foraging areas and overall flew lower than the operating range of most turbine blades, suggesting a lower collision risk. Flight heights of bats suggest that during summer the risk of collision with wind turbines was high for most studied bats at the majority of currently installed wind turbines. For siting of wind parks, preferred bat habitats and commuting routes should be identified and avoided.