Paper Review – Habitat use by Noctule bats in a landscape with a high density of wind turbines

Paper Review – Habitat use by Noctule bats in a landscape with a high density of wind turbines

In this article, Peter Shepherd (BSG Partner) reviews a recent study that looked at noctule ranging behaviour in an area of high wind turbine density in Germany, and draws conclusions with regard to how the study can potentially be used to inform wind farm surveys, assessment and mitigation strategies.

Background and Context

In July 2016 the research team at the Leibnitz Institute, in association with Tel Aviv University, published the results of a study of habitat use by noctule bats Nyctalus noctula with particular reference to wind turbines in north eastern Germany[1].

The study is of particular interest in that it used miniaturised GPS tracking devices to monitor bat flight paths, thus enabling data on bat location, height and speed to be derived at a detail not possible using traditional VHF radio tracking methods improving our ability to assess the impacts arising from wind farm and other developments. The study investigated possible differences between male and female flight behaviour and the distance this highly mobile bat species covers in an evening.

Given the lack of available studies on noctule to inform the Bat Conservation Trust’s Core Sustenance Zone (CSZ)[2] advice, the study is helpful as it extends our understanding of how noctules use the landscape and the distances they travel from roosts. In this article Dr Peter Shepherd provides a summary of the study and its key findings and considers some of the questions arising from the research in relation to working with noctules and assessing potential impacts of wind farms alone or in combination.

Overview of Research

Study Area

The study was undertaken in the Carmzower Forest area in the Uckermark area of north eastern Germany. The landscape is dominated by intensive arable farmland with small patches of woodland, areas of pasture that are often associated with small settlements, and a large number of open water bodies (groove lakes and kettle holes) created by glacial activity. This study area also supports a high density of wind turbines.

Research Methods

The research involved the capture of 3 female and 5 male noctule bats and the subsequent monitoring of their flights from day time roosts. The miniaturised GPS loggers were attached to each bat using custom built collars and skin bond glue and GPS positions were recorded every 30 seconds. The advantage of using these GPS devices meant that as well as recording the location of the bat at very regular intervals, the data also enabled the research team to measure the height at which bats flew and to calculate flight speed. The male bats were monitored in early summer (May/June) and the females were monitored in mid-summer (July). Each bat was monitored for between 1 and 5 days either shortly after sunset and/or before sunrise.

Results

The study revealed that the 8 bats studied travelled on average between 14 and 26.6 km during evening flights. Average flight durations varied between 61 and 105 minutes. Individuals typically ranged 5.8km (+/- 2.9km) from the day roost, although one bat was recorded 13.7 km from the roost.

Bats were recorded flying at heights between several metres above ground to over 250 metres above ground level, and typical flight speeds ranged between 4.2 and 6.2 m/s (15 – 22km/hr). Habitat preferences indicated that bats were less likely to be found over conventional agricultural farmland and forest than would be expected given the availability of the habitat in the landscape. Bats were recorded more frequently than would be expected over organic crops, grassland, urban areas and wetland, with wetland being particularly preferred.

Flights by bats before sunrise were shorter in duration and distance travelled than those undertaken in the evening. Morning flights lasted on average 22 minutes (+/- 12 minutes) and covered 5.8 km (+/- 3 km).

The study recorded notable differences in flight behaviour and habitat use between male and female bats. Males typically flew shorter distances, at lower heights and 1.5 times faster compared to female bats. Females were less likely to spend time over bushes and hedges than would be expected given the extent of this habitat, whereas males were more likely to be recorded above this habitat type than would be expected. For the 8 bats studied both males and females preferred wetland and organic cropland. Males were less likely than females to be recorded above conventional cropland and more likely than females to be found over forest, scrub and hedges, organic cropland, grassland and wetland.

Discussion of Results

The study discusses the results with particular reference to wind farms. A number of wind farms are present within the study area and bats were recorded flying through and around fields containing turbines. The males tracked in this study did not frequently forage close to wind turbines or cross fields containing them. It is suggested this may be because these bats had established flight routes to key foraging grounds. However, the authors speculate that there may be a risk to juvenile male bats as they establish flight paths in their first year. The study also considers whether the males are avoiding the turbine field and if so raises concerns about habitat displacement caused by turbine fields

In contrast, female bats flew closer to turbines than expected. Two bats were recorded foraging for several minutes within several hundred metres of turbines and one bat was recorded flying through the turbine field at two different wind farms. More than a quarter of female flights over open habitats were recorded at heights between 70 and 130 metres, which is the height range typically used for wind energy generation in the study area. Given the flight behaviour of females the authors consider they are at risk of mortality.

The authors of the paper discuss the differences observed between male and female bats but acknowledge that these may not be solely attributable to gender but may reflect seasonal differences in behaviour or even a combination of both factors.

Viewpoint

Key Findings and Limitations of the Work

The study provides some very useful data and insights into noctule behaviour. It suggests there may be differences between male and female bats, although it is acknowledged that these effects may be driven by seasonal variation rather than the gender of the bats.

Not all the bats included in the study flew through turbine fields, but a number were recorded flying through wind farms potentially putting them at risk of mortality. The paper does not provide data on prevailing weather conditions on the nights of the study and as such it does not discuss how weather and in particular wind speed may influence noctule flight behaviour during the study period.

One interesting question that arises from this study is whether individual noctule bats remain faithful to particular flight paths or foraging areas throughout the year, or if they vary flights based on food availability, season or prevailing weather conditions. If the noctules in this study do remain faithful to flight paths and foraging areas through the active season then predicting risk of mortality would be more reliable. However the constraints of using the GPS loggers did not allow for longer term monitoring of individual bats.

The study did not extend into the late summer/autumn period when most collisions (90%) are reported to occur in Europe[3]. As such it is unknown whether flight behaviour changes again at this time that might shed light on the higher rate of mortality reported at this time of year.

The study confirms, as would be expected, land-use influences noctule flight paths and habitat usage. Wetland and grassland in this study as well as urban fringe were preferred habitats, with conventional arable land being generally avoided as a foraging resource. This chimes with other studies referenced in the paper and with observations by BSG Ecology in particular in regard to wetlands including agricultural reservoirs and drainage ditches in otherwise intensively farmed landscapes in lowland England. The study also suggests that organically farmed land may also provide greater foraging resources for noctules. This appears to support the general findings of a study of bat use on organic farms in the UK[4] although in that study noctules were primarily recorded from pasture and water bodies with an apparent preference for these habitats within organic farms. Noctules were rarely recorded over conventional arable habitats in this study and not at all over organic arable habitat.

Implications for Wind Farm Survey, Core Sustenance Zones and Cumulative Assessment

The study raises questions about how we survey for noctules in relation to development. Wind energy sites and other developments are likely to only ever represent a small proportion of the overall habitat used by animals from a particular species and a particular colony, which can make assessing the potential impact on a colony and its conservation status difficult to determine. The study confirms the importance of identifying flight paths and key foraging areas at proposed wind farms when considering potential impacts on bats.

It also suggests that additional information on the number of animals recorded at any one site and how they are using it (commuting or foraging and height of flight) may need to be gathered to better assess likely risks of mortality? This additional data (when compared to standard acoustic registrations from automated detectors) to standard acoustic registrations from automated detectors can be difficult and potentially disproportionately expensive to obtain. Use of miniaturised GPS loggers, which are currently not readily available to the commercial market and more traditional radio tracking studies are dependent upon knowing where the noctules are roosting so they can be caught and monitored. Often this is not feasible as bats are flying in the open at height at sites where development is proposed making them almost impossible to capture and roost locations are not known and could be several kilometres away. Other techniques such as the use of thermal imaging cameras may therefore need to be considered to supplement more established / conventional survey, as they would at least allow numbers and behaviour of bats to be characterised more accurately.

The study also provides valuable information on flight distances and duration, which can potentially be used to inform mitigation strategies to avoid or minimise collision rates at wind farms and other developments either alone or in-combination. The Bat Conservation Trust (BCT) survey guidelines[5] promote consideration of Core Sustenance Zones (CSZ) for impact assessment and encourage assessments to consider all impacts that may arise on the CSZ of a colony. Data for noctule however are limited and the CSZ for noctule in the survey guidelines is based on one study and proposes a CSZ of 4km for noctule that is derived from mean maximum foraging distances. The data presented in this study adds to our knowledge base and suggests that CSZ for noctule may need to be extended to at least 6 km.

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[1] Roeleke, M., Blohm, T., Kramer-Schadt, S., Yovel, Y & Voight, C.V. (2016).  Habitat use of bats in relation to wind turbines revealed by GPS tracking. Sci. Rep. 6, 28961; doi: 10.1038/srep28961 (2016).

[2] The area surrounding the roost within which development work can be assumed to impact the commuting and foraging habitat of bats using the roost, in the absence of information on local foraging behaviour.

[3] Rydell, J., Bach. L., Dubourg-Savage, M.J., Green, M., Rodrigues, L & Hedenström, A. (2010). Bat mortality at wind turbines in northwest Europe. Acta Chiroptera 12

[4] Wickramasinghe, L.P., Harris, S., Jones, G & Vaughan, N. (2003). Bat activity and species richness on organic and conventional farms: impact of agricultural intensification. Journal of Applied Ecology Vol 40: 6. Pp 984-993

[5] Collins, J (ed). (2016), Bat Surveys: Good Practice Guidelines, 3rd Edition, Bat Conservation Trust, London.

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