Collision risk of bats with small wind turbines: Worst-case scenarios near roosts, commuting and hunting structures.

Small wind turbines (SWTs) have become increasingly common within the last decade, but their impact on wildlife, especially bats, is largely unknown. We conducted an operational experiment by sequentially placing a mobile SWT with five different operational modes at six sites of high bat activity, including roosts, commuting structures, and highly frequented hunting areas. Bat flight trajectories around the SWT were documented at each site during five consecutive nights using a specifically designed high-spatial-resolution 3D camera. The recordings showed high bat activity levels close to the SWT (7,065 flight trajectories within a 10-m radius). The minimum distance to the rotor of each trajectory varied between 0 and 18 m, with a mean of 4.6 m across all sites. Linear mixed models created to account for site differences showed that, compared to a reference pole without a SWT, bats flew 0.4 m closer to the rotor (95% CI 0.3-0.6 m) if it was out of operation and 0.3 m closer (95% CI 0.1-0.4 m) if it was moving slowly. Exploratory behavior was frequently observed, with many bats deviating from their original flight trajectory to approach the rotor. Among 7,850 documented trajectories, 176 crossed the rotor, including 65 while it was in motion. The collision of one P. pygmaeus individual occurred during the experiment. These results demonstrate that, despite the generally strong ability of bats to evade moving rotor blades, bat casualties at SWTs placed at sites of high bat activity can reach or exceed the current threshold levels set for large wind turbines. As SWTs provide less energy than large turbines, their negative impact on bats should be minimized by avoidance measures such as a bat-friendly site selection or curtailment algorithms.

Introduction

Reducing carbon emissions is a major goal within worldwide efforts to mitigate global climate change. As a consequence, the renewable energy sector has rapidly expanded within the last decade, with wind power being among the most widely used forms of alternative energy [1,2]. Despite their positive effects on the climate, wind farms can severely impact the local fauna [3], especially bats and birds [4]. Indeed, direct collisions with rotor blades and, to a lesser extent, barotrauma account for a significant proportion of bat deaths [5-7]. The particular vulnerability of bats is in part due to their relatively low reproductive rates and their extended lifespan [8,9]. Population viability studies have shown that the collisions of bats with turbines can lead to plummeting population sizes [10-12] and have stimulated debate on whether priority should be given to climate or to species protection [13].

Although previous studies of bat collisions predominantly focused on large wind turbines, the installation of small wind turbines (SWTs, typically < 15 kW power, < 30 m hub height) has increased, particularly in the private sector, but their effects on wildlife is thus far unknown [14]. Extrapolation of the findings of studies on large wind turbines to SWTs is problematic for at least two reasons: First, bat fatalities are influenced by tower height, rotor size, and revolutions/min, all of which differ significantly between small and large wind turbines [6]. Second, unlike large wind turbines, SWTs are often placed much closer to the homes, farms, or factories of their owners and therefore in closer proximity to hedgerows, gardens, buildings, and other habitat structures known to be attractive to bats. As environmental impact assessments are typically not required for SWTs [14], their damage to bat populations has yet to be assessed.

Preliminary studies have found evidence of negative effects of SWTs on bats, albeit less severe than those of large wind turbines [15-17]. However, those studies addressed only the long-term effects, as all of the included SWTs had already been operational for several years prior to the study period. Since bats tend to increase their exploratory behavior when confronted with new objects but make use of longer echolocation intervals in known vs. unknown areas [18], the installation of a SWT within a formerly undisturbed bat habitat may result in a collision risk that is initially high but then decreases as the SWT becomes known to the bats. Mortality rates derived in long-term studies may therefore have been underestimated. More than half of all European bat species inhabit maternal roosts in buildings, while other species hunt preferably at street lamps, in gardens, or cattle sheds [19]. Given this inherent overlap between SWT sites and sites frequently used by bats, studies addressing the initial collision risk of bats with SWTs are urgently needed.

The aim of our study was to simulate a worst-case scenario to estimate the maximum initial damage that an SWT could impose on the local bat fauna. Specifically, we were interested in the number of exploratory flights, rotor passes, and subsequent collisions of bats when confronted with SWTs newly installed in bat hotspots. We therefore sequentially placed a mobile SWT at six different sites of high bat activity, i.e., located close to vegetation or other structures of interest, and then monitored bat behavior for five consecutive nights using a stereo-optical infrared camera system and acoustic records. By conducting an operational experiment with the SWT at different rotor speeds we also provide knowledge on how bat behavior around SWTs is influenced by the different operational modes of the turbine. In addition, we examined species-specific differences in maintaining distance from the SWT, in order to gain further information on the respective differences in the collision risk.

Material and methods

Study sites

Six sites with high bat activity were selected based on their proximity to roosts, commuting structures along which bats commute between roosts and foraging sites, or highly frequented foraging sites. All of the sites were located in southwestern Germany, between the city of Freiburg and the Rhine River (Fig 1), and separated from each other by a distance of up to 40 km. The landscape structures at the sites included forest edges, hedgerows, riparian forests, and meadows of high insect density.

Fig 1

Site selection.

Overview of the six sites (a–f) used in the experimental set-up.

Experimental design

Between May and August 2018, an operational experiment was conducted at each of the six sites during five consecutive evenings/nights, from 0.5 h before to 2.5 h after sunset. On the first night, bat activity was monitored acoustically and visually in the absence of a SWT. Instead, a reference telescope pole set up at the same position allowed acoustic monitoring at the same height (4 m) as subsequently used for the SWT but also served as a reference in distance measurements of bat flight paths during the first night. On all following nights, a mobile SWT (turbine type WKA 600, EAN 4251108115622, HeuSa GmbH, rotor diameter 2.8 m, nacelle height 6.1 m) was set up, separated from the relevant vegetation structures or buildings by only a few meters (Fig 1). The generator was replaced by a motor (MS 803–0,75 kW-6pol-B3, JS Technik GmbH) to allow the manipulation of rotor speeds. On the second and third nights, the rotor was either propelled at full power or held in position by brakes. This was done to assess whether the collision risk was higher when the SWT was immediately operated at full speed than when the bats were allowed an initially safe encounter and were thus able to adapt to the newly introduced, unknown object. On the fourth night, the SWT was propelled at half of its full power and on the fifth night operational modes that had been prevented on the previous days due to adverse weather conditions were tested (for details on the experimental schedule, see S1 Table).

The speed of the rotor blades was 88 rpm and 102 rpm at half- and full-motor power without wind, respectively. Two further rotor speed classes resulted from additional acceleration by wind, such that the experiment consisted of six possible operational modes: 1) no SWT, 2) SWT out of operation, and 4–6) four different rotor speed classes (class 1: 60–90 rpm, 32–50 km/h at the outer blade tip, class 2: 96–115 rpm, 51–60 km/h, class 3: 116–135 rpm, 61–71 km/h, class 4: > 135 rpm, >72 km/h.)

In addition to video recordings, the rotor was constantly monitored by at least one person using a thermal camera. In case of a collision, the SWT was immediately halted and the experiment terminated ahead of schedule at the respective site. Thus, there was a risk of a maximum six bat casualties in total across all sites. This risk was acknowledged in the permit granted by the Regional Council Freiburg (05.04.2018, file reference: 55–8852.44/100), which also confirmed that our experiment complied with the federal law on nature protection of the European Union and that there would be no negative effects on the bats at the population level.

Bat acoustic activity

Bat acoustic activity was monitored for use in species identification and was automatically recorded using two electret ultrasound microphones (Knowles FG, Avisoft Bioacoustics) placed at 4 m height on either the SWT or the telescope pole and facing opposite directions to allow the omnidirectional recording of bat calls. The latter were evaluated by combining automatic classification and manual post-validation [20] using the software BATscreen V 1.0.5 (bat bioacoustictechnology GmbH, Winkelhaid, Germany).

Three-dimensional recording of bat activity using a stereo-optical infrared camera

Bat activity was recorded three-dimensionally using a stereo-optical system that is part of a multisensor array still under development [21-23]. The development goals of the system are the visual detection and localization of bats during extended periods of time while keeping the associated costs to a minimum. The two infrared-sensitive cameras employed in the study each contained a 5MP 1/4” CMOS sensor OmniVision OV5647 and were controlled by a single-board computer (Raspberry Pi). Images were obtained at a frequency of 15 per second and stored on usb flash drives. To ensure matching timestamps, the system was equipped with a GPS module (Adafruit Ultimate GPS HAT). The cameras were positioned at a distance of 11–14 m from the turbine footing. Two infrared spotlights (IR06/60 850NM, Indexa GmbH) were placed at a distance of 2–3 m away from but facing the SWT. The system allowed for the detection of bats up to 20 m away.

The three dimensional coordinates of the bat positions were calculated using a homogeneous DLT (direct linear transformation) method that included several triangulation and calibration steps (see [25] for details). Each flight trajectory (the unit of replication used in the statistical analysis) consisted of all coordinates of the bat between its first and last appearance in the filmable cone-shaped area around the SWT. Different flight trajectories therefore do not necessarily correspond to different individuals. The minimum distance to the rotor was calculated as the point of each trajectory with the shortest distance to the area swept by the rotor. For the reference pole, a hypothetical disk with the same diameter as the SWT rotors was used.

Habitat and weather data

Wind speeds were measured every minute using wind loggers (PCE-ADL 11, PCE instruments) installed at nacelle height. Temperature and humidity were recorded every 15 min at a height of 1 m from the ground (EasyLog EL-USB-2, LASCAR Electronics). Brightness was derived from the global solar radiation at DWD (Deutsche Wetterdienst) station 1443, Freiburg. This parameter was used in the statistical models to investigate whether bats behaved differently before total darkness set in, when, in addition to echolocation, optical orientation was possible.

Statistical analyses

Statistical analyses were conducted using R 3.5 (The R Foundation for Statistical Computing). Linear mixed effect models (function lmer from R-package lme4 [24]) were constructed to evaluate the effect of operational state and global radiation on the minimum distance of each flight trajectory to the rotor blades. Global radiation was included to assess whether bats maintained a larger distance from the SWT close to twilight, when visual recognition was still possible, than after dark, when echolocation was the sole means of orientation. SWT site was included as a random factor to account for seasonal and site-specific differences in bat activity and species composition. The models were evaluated visually by plotting their residuals against the fitted values and the residual quantiles against the quantiles of a normal distribution. Precipitation, wind speed, and temperature were removed from the final model because of their low variation in the data (mostly no wind and temperatures between 15 and 20°C) and the lack of data at one site, which would have prevented a joint analysis (prior models including environmental parameters showed similar results). The effect of species or species group (e.g., Myotis) on the minimum distance was analyzed for a subset of flight trajectories for which correct species identification was possible.

Results

Bat species and activity levels

Species composition differed markedly between sites depending on the roosts or hunting sites in their vicinity. Commuting activity of 20 and 50 Pipistrellus pipistrellus individuals was observed at two sites (a and c), and of >20 Myotis mystacinus individuals at one site (c). Site d was less than 20 m apart from a maternal roost of >300 individuals of P. pygmaeus, 20 individuals of P. pipistrellus, and <5 individuals of P. nathusii. The activity level of all three species was high, particularly that of P. pygmeaus. Sites b and e were characterized by the high hunting activity of M. myotis and M. nattereri as well as P. pipistrellus. Eptesicus serotinus hunted frequently at site a, and Nyctalus noctula and N. leisleri sporadically at sites b, e, and f.

At the six sites, a total of 7,850 flight trajectories were filmed in high three-dimensional resolution during 84 h of video documentation (Fig 2). Since each bat could leave more than one flight trajectory by exiting and then re-entering the camera’s image frame, the number of flight trajectories does not necessarily correspond to the number of filmed bat individuals. Among the recorded flight trajectories, 90% were within 10 m distance to the nacelle.

Fig 2

Example of a bat passing the rotor blades of a small wind turbine at close distance.

Bat behavior around the rotor blades

High bat activity levels (7,850 flight trajectories) were documented within close distance to the SWT (10 m radius). Exploratory behavior was frequently observed, with many bats appearing to deviate from their original flight trajectory (even while commuting) to approach the rotor. The minimum distance of the filmed trajectories to the rotor varied between 0 and 18 m, with a mean of 4.6 m (± 2.6 m) across all sites. The results of the linear mixed model showed that, compared to the distance to the reference pole (7.78 m; 95%CI: 6.6–9 m), the bats approached the rotor 0.4 m closer if it was out of operation (95% confidence interval [95%CI]: 0.3–0.6 m) and 0.3 m closer if it was moving slowly (rotor velocity class 1, 95%CI: 0.1–0.4 m) (Fig 3 and Table 1). The distance of the bats to the rotor when operating at higher velocities was similar to that maintained to the reference pole (0.06–0.11 m). The bats remained farther away from the SWT as long as they could still see it than when they had to use echolocation to navigate in increasing darkness.

Fig 3

Results of the linear mixed model assessing the minimum distance to the rotor dependent on the operational state of the SWT and the global radiation.

The monitoring site served as a random factor. Circles indicate regression coefficients, and lines the 95% confidence intervals. The telescope pole (dotted line) was set up as a reference.

Table 1

Test statistics for Fig 3.

	Estimate	Lower 95% CI	Upper 95% CI
Ø telescope pole	7.78	6.56	8.99
SWT no rotor movement	-0.45	-0.61	-0,28
Rotor velocity 1	-0.31	-0.49	-0.12
Rotor velocity 2	0.06	-0.12	0.23
Rotor velocity 3	0.09	-0.22	0.41
Rotor velocity 4	0.11	-0.12	0.33
Log global radiation	1.01	0.71	1.30

N = 7.850, R² fixed effects = 0.01, R² random effects = 0.17.

The number of paths that crossed the rotor diameter was calculated for each operational mode (Table 2). For the telescope pole, a hypothetical rotor was substituted. When the SWT was out of operation and the rotor blades were not moving, between 2 and 46 flight trajectories crossed the rotor, corresponding to 1–8% of all documented trajectories. When the rotor was in motion, the number of crossing trajectories decreased to 0–6, depending on the rotor velocity and the site. At four of the six sites, no rotor passes were documented with the rotor operating at low speed (rotor class 1), but at five of six sites sporadic crosses occurred even at a high rotor speed. Overall, of the 7,850 documented trajectories, 176 crossed the rotor, including 65 while the rotor was in motion.

Table 2

Flight trajectories crossing the rotor blade diameter for the different operational modes and sites.

Site	No SWT	SWT out of operation	Rotor velocity class 1	Rotor velocity class 2	Rotor velocity class 3	Rotor velocity class 4	Sum
a	9/4.1	2/1.6	5/3.8	9/6.4	1/7.7	1/1.6	27/4.2
b	2/4.1	46/8.2	0	0	6/4.3	2/4.1	56/3.5
c	9/2.6	11/1.9	13/1.5	14/1.7	1/1.5	2/0.4	50/1.6
d	3/2.8	2/1	0/0	3/1.6	4/5.7	0/0	12/1.9
e	7/1.3	8/1.9	0	0	0	0	15/0.5
f	5/2.4	7/2.1	0/0	4/1.6	-	-	16/1.5
Sum	35/2.9	76/2.8	18/0.9	30/1.9	12/3.8	5/1.2	176/2.2

The absolute number/percentage of all recorded flight paths is shown.

The sites differed markedly in vegetation structure, vicinity to roosts, commuting structures, and species range. This variation was mirrored in the results of the statistical analyses, in which site, as a random factor, accounted for more variation (R² = 0.17) than did the fixed effects (R² = 0.01). The influence of species composition became apparent in a separate analysis of a subset of flight trajectories (n = 1,158) by species that could be identified acoustically:

Hunting or commuting by individuals of the Myotis group is generally more structure-oriented than hunting by pipistrelles or even the Nyctaloid group, which preferably hunts in open space. This was reflected in our species-specific model, which showed that Myotis individuals approached the rotor 0.5 m closer than Pipistrellus individuals whereas Nyctaloid individuals remained 0.6m farther away than pipistrelles (Fig 4 and Table 3).

Fig 4

Results of the linear mixed model assessing minimum distance to the rotor dependent on the bat species.

The respective site served as a random factor (n = 1,158). Circles indicate regression coefficients, and lines the 95% confidence intervals. The Pipistrellus group (dotted line) was defined as the reference.

Table 3

Test statistics for Fig 4.

	Estimate	Lower 95% CI	Upper 95% CI
Ø minimum Distance Pipistrelloids	4.57	3.75	5.39
Myotis sp.	-0.54	-0.90	-0,17
Nyctaloids	0.57	0.03	1.11

N = 1,158, R² fixed effects = 0.01, R² random effects = 0.17.

Collision risk

One collision, of a P. pygmaeus individual, was documented at site e on November 8, 2018 at 23:30, during the fifth night of the experiment and shortly before the official schedule ending (S1 Video). The rotor blades were moving at ~45 km/h (speed of rotor blade tip, rotor speed class 1). The bat, which presumably died on impact, was catapulted into the upper ends of the nearby hedgerow (see Supplementary material). Unfortunately, it could not be located, despite immediate extensive search efforts using flashlights and thermal cameras. Searches early the next morning were also fruitless, possibly because the cadaver had already been removed by a predator.

Discussion

Bat activity levels and behavior

Bat activity levels were high at all six study sites in Southwestern Germany, with a total of 7,850 bat flight paths in close proximity to the rotor filmed during 5 days. By contrast, an operational experiment in Northern Germany conducted at 20 SWTs of ~20 m height reported ~500 flight paths during 24 nights with only 41 of the flight path approaching the rotor closer than 10 m [25]. Bats showed little exploratory behavior in the Northern Germany experiment and only one rotor pass (when the SWT was out of operation) whereas our recordings revealed frequent exploratory behavior and 176 rotor passes (65 while the rotor blades were in motion).

The striking differences between the two studies can be attributed to several factors: First, both bat activity levels and species number seem to be lower at SWT sites in Northern Germany than at those in other regions in Germany, where a larger number of acoustic recordings have been documented using the same technical settings [25,26]. Whereas Nyctaloids, especially E. serotinus, dominated in the Northern Germany study, the sites in Southwestern Germany chosen for placement of the mobile SWT were close to the roosts and commuting structures of more structure-oriented species such as those of Myotis or Pipistrellus. Our results showed that the general structural affinities of these groups are reflected by the distance from the rotor maintained by their individual members. Second, the SWT in our study was only half the height of the SWT assessed in Northern Germany (~ 20 m). The larger overlap of SWTs with the activity range of most bats, especially those within the structure bound Myotis and Pipistrellus groups, would result in much higher contact probabilities. Third, and most important, the sites in Southwestern Germany were explicitly chosen for their high bat activity levels and thus simulated a worst-case scenario. The much higher bat activity levels in our study than in the study from Northern Germany were therefore not surprising, although the 200-fold difference was unexpected.

The higher rate of exploratory behavior can also be explained by our having documented bat behavior directly after installation of the SWT, whereas in Northern Germany the SWT had already been in operation for several years. Bats tend to be curious about new objects, which would presumably include a SWT newly positioned in a formerly undisturbed area. However, in contrast to the SWT, the telescope pole was not explored, probably because its long, thin structure resembled that of vegetation such as dead trees or other common landscape features. Bats approached the rotor 0.4 m (95%CI: 0.3–0.6 m) closer than they did the reference pole when the blades were moving slowly, an indication that moving objects are of greater interest than immobile objects. At a higher rotor velocity, the distance maintained by the bats to the SWT and the reference pole was similar. This behavior may have been due to a disruption of echolocation calls with increasing rotor speed [27,28], which hindered a closer examination by the bats.

Among the 7,850 flight paths documented in close proximity to the SWT, only one resulted in a collision. The number of flight paths that crossed the rotor blades was small (n = 176), and only a third of those were documented when the rotor was in motion. However, over the 5 days of the experiment, the rotor blades were moving for a total of only 47 h. Thus, although all but one of the bats that crossed the moving rotor blades during the study were able to avoid a collision, longer experimental time frames and a continuously running SWT would likely have resulted in a larger number of fatalities. A relatively low risk of collision between bats and SWTs was reported in a 2-year study, during which time no carcasses were found at 21 SWTs [16], whereas in a 3-year study up to three carcasses were found at 31 SWTs [15]. A questionnaire study involving 271 SWT owners yielded reports of three carcasses in total, corresponding to an estimated annual rate of 0.008–0.169 collisions per SWT [16]. Carcass searches with trained dogs delivered a similarly low mean collision rate of 0.81 dead bats per SWT/year, but in a worst-case scenario the rate may be as high as 15 dead bats/year per SWT [15]. A Swiss study from 2016 reported eight collisions between July and September at three SWTs located in an industrial area [29], thus demonstrating that these worst-case scenarios can become reality. As in the Swiss study, the collision in our experiment occurred at the end of the maternal roost phase, when newborn bats begin to accompany adults on their flights. The killed bat may therefore have been a newborn that lacked the maneuverability of an adult, but since the carcass could not be located this could not be confirmed.

It should also be pointed out that to conduct a worst-case experiment we explicitly chose high-risk sites, i.e., those with exceptionally high bat activities. In addition, SWTs would normally be placed at greater distance to buildings or vegetation, for better wind exposure. Moreover, the rotor blades of our SWT were accelerated during calm nights by a motor, which would not be done under real-life operating conditions. Accordingly, the collision risk at standard SWT sites will most likely be lower than in our study.

Methodological considerations and study limitations

The transferability of our results to other local and structural conditions, bat species, and SWT types is limited. Despite the generally broad species richness in our study, with at least nine different species from the pipistrelle, Nyctaloid, and Myotis groups, rare species such as the barbastelle bat were not assessed. Also, due to logistical limitations, our mobile SWT, with its height of only 10 m and a rotor diameter of 3 m, was one of the smallest SWTs available. An extrapolation of our findings to larger SWTs or other SWT types such as those with vertically moving blades is not possible.

Interestingly, much of the variation in our data was explained by the SWT site, which differed with respect to vegetation type, vegetation height, bat activity pattern (roost, commuting, hunting), bat species, and landscape variables (proximity to settlements, forests, or agricultural land). Further controlled experiments focusing on the impact of single-site factors would improve the predictability of bat activity and collision risks at SWTs.

Conclusion

Our study showed that SWTs situated near roosts, commuting or hunting structures will result in high bat activities and exploratory behavior within close proximity to the rotor. This will not necessarily lead to high casualty rates, as most bats in our study were adept at avoiding the moving rotor blades. However, the collision of one P. pygmeaus individual at one of the six experimental sites demonstrates that caution is warranted, especially if the potential cumulative effects over long operational periods are considered.

Although environmental impact assessments are typically not required for SWTs, their operation must still comply with the federal law on nature protection of the European Union; that is, SWT operation must not pose an increased mortality risk for bats. While our study design provoked a higher collision risk than would be expected under standard SWT conditions, the threshold currently applied to large wind turbines of 1–2 dead bats per wind turbine and year could conceivably also be reached by SWTs. However, as SWTs have a much lower energy output than large wind turbines and their cost-benefit is accordingly lower, specific regulations that take these factors into account are needed.

We therefore suggest bat-friendly SWT site selection, in which proximity to structures of particular interest to bats, such as buildings, trees, forest edges, hedgerows, and waterbodies are avoided. If this is not possible, such as with SWTs designed for installation on rooftops, curtailment algorithms during periods of high bat activity will decrease the collision risk for bats at SWT sites.

Overview over the different operational modes of the turbine applied to each site.

* = Experiment stopped ahead of schedule due to bat collision.

(DOCX)

Click here for additional data file.

Documentation of the collision of P. pygmaeus during the experiment.

(AVI)

Click here for additional data file.

13 Apr 2021

PONE-D-20-40193

Collision risk of bats at small wind turbines- worst-case scenarios near roosts, transfer or hunting structures

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Reviewer #1: 1. English needs a thorough review. Grammatical errors and inelegancies are common and must be fixed.

2. Remove speculations and lengthy details

3. Why no comparison with the results of the British studies?

Reviewer #2: Summary

In this experiment, the authors examined the proximity to which bats approach small wind turbines (SWT) by temporarily installing a turbine at six different sites and employing different operational treatments. I think the key interesting result here is that bats appeared to approach SWT more closely than a pole used at the same site as a reference point, but only when it was not operational at all, or the blades were moving slowly. However, whilst the authors discuss the results of the treatment where the blades were moving slowly, I couldn’t see any arguments for why a turbine and a vertical pole were approached differently if the blades were not moving at all – it would be interesting to consider what kind of behavioural mechanism may result in this finding.

Whilst I felt there were some interesting results in this study, I have a number of concerns, primarily:

• The relevance of the study to realistic installation scenarios. Were the installations following any guidelines? From the photos provided some of these locations did not look very suitable for turbines (in terms of laminar flow and maximal wind speed), and are not in accordance with the Eurobats guidance on locating small wind turbines.

• The wording of the aims are very vague (L82 – 89) – it needs to be clearer what is actually being measured that will enable the aim to be addressed. What does damage mean? Also, I think the bats’ response is going to vary so much by scenario there's no way of knowing whether this is max damage or not.

• Results felt quite fragmented in places and lacking key information – findings from a linear model are presented but with no test statistics and with the results spread over a large area e.g. the headline result is given on one page but we don’t find out until the following page that the ability of the model to explain the variation in the dataset is tiny (R2 of 1%).

• The final conclusion (L28-30) of the abstract is that casualties can reach relevant levels over time. I do not feel the authors have the data to make this claim, and no information is presented on what constitutes a “relevant level”. Bats were monitored behaviour over 5 days in areas of purposefully high activity – we don’t know what would have happened if the turbines had been in place for longer.

Ethics

I found it very surprising that the institute the researchers are based at does not have a process for reviewing potentially harmful research activities. The fact that a bat was killed as a result of this work demonstrates this – this isn’t really a criticism of the nature of the research as there’s clearly a role for these sorts of experiments. However, there is certainly a conversation to be had about what is acceptable risk and what might be considered unethical. Readers may take a variety of views so some consideration in the discussion (or an expansion on the existing text), over and above “it was legal” is warranted.

General

Detailed comments

L24: The term reference pole is introduced here with no context – a little rephrasing here would clarify the purpose of the pole.

L31: The last line of the abstract felt a bit abrupt – a little rephrasing/expansion/clarification is needed.

L67: the authors consider that a major shortcoming of previous research is that the turbines had been up for longer time periods than in this study, meaning that the immediate effects could not be addressed. I agree that it’s really interesting to look at whether effects change over time but feel that this is only a shortcoming if your main interests are quantifying short term effects. If you’re interested in what happens over the longer term, this is not a short coming at all. I would rephrase to reflect this.

L74: is there a ref to back up the statement that the “preferred installation [of SWT] in close

proximity to the buildings which they supply with energy”? Suitable locations for SWT need to avoid areas where barriers (such as buildings, treelines) can disrupt wind flow. Indeed this is why turbines are no longer installed in urban areas because the pattern of wind flow is not suitable. Most of the SWT I am aware of today are installed in fields, away from buildings.

L82: not sure “unravel” is the most appropriate word to use here – quantify/characterise?

L85: change acoustical to acoustic

L92: I am not familiar with the term “transfer structures” (and at L177 “transfer activity”) – some clarification or rephrasing needed. I presume this means commuting to foraging sites?

L100: We need more information on the expt set up e.g. distance to buildings/vegetation etc. How far away was the reference pole to the turbine?

L128: We need some information on what these acoustic data were used for? I was expecting to see an analysis of activity at the turbine vs. ref pole, but I think it was to obtain species id for some of the trajectories?

L138: We need information here on the metrics being quantified e.g. minimum distance

L160: It would be helpful if the stats could refer back to the specific objectives being addressed and in the same order as the introduction. Is the unit of replication here the flight trajectory? Some discussion that there will inevitably be pseudo-replication in the analyses since it will not be possible to distinguish individuals. Also, in the results there is a comparison of the turbine and the ref pole but there is no explanation here on how or why they are doing this.

L164 & 203: these sentences sounds awkward and need rephrasing

L194: Was there a reason for picking 10m here as the distance? I realise any distance picked might be arbitrary but if there was a rationale for the 10m it would be useful to clarify here.

L197: These are the key results but there is much which I found unclear here. Are these - are these modelled estimates or raw data? Where are the results of the model and the test statistics? We find out on the next page that R2 for fixed effects is only 1% - I found this quite hard to reconcile with the confidence intervals shown in Fig 3.

L215: the number of bats passing in front of the rotor when it’s not moving doesn’t seem like a very relevant metric?

L225: Some of the sites that differed in vegetation structure etc also differed in species composition so I just don’t feel there is sufficient replication to tease this all out

L265: I’m not familiar with the term “structure bound” for bats? Pipistrellus sp are also known to forage along forest edge and interiors, and are closely associated with buildings.

L279 (and abstract): these point estimates should only be given with the confidence intervals.

L284: comment is made that no habituation over the 5 days was evident but I think it’s important to note that the treatments changed each night so this doesn’t seem very surprising.

L294: 64 of 65 trajectories (not bats)

L347: interestingly, this siting recommendation was also made by Minderman et al. but for opposite reasons (because of the apparent disturbance / avoidance effect detected in that study).

**********

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7 May 2021

Dear Prof. Brock Fenton,

Thank you very much for the great opportunity to publish in Plos One! We are greatful for the constructive comments from you and both reviewers and address each point separately (see below, printed in black, for better readability we coloured your and the reviewers remarks in green- of course works only in the attached word document). We changed all formatting according to your comment and the figures have been checked with pace. We also sent our manuscript to a professional proof reading service to comply with Reviewer 1´s comment on the quality of our English. We have left also the English correction visible with track changes. Our proofreader suggested also a different wording in the title, we hope that is okay? Otherwise please feel free to not accept the changes.

The changes we conducted as a response to the Reviewer´s comments are addressed with line numbers which correspond to the “track changes” manuscript version so they can be easier found.

We hope our revised manuscript now merits a publication in PlosOne.

Best regards,

Stefanie Hartmann

PONE-D-20-40193

Collision risk of bats at small wind turbines- worst-case scenarios near roosts, transfer or hunting structures

PLOS ONE

Dear Dr. Hartmann,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by May 28 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

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If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

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We look forward to receiving your revised manuscript.

Kind regards,

Brock Fenton

Academic Editor

PLOS ONE

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Point adopted

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Point adopted

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All above formatting changes have been conducted without tracking changes for better readability.

Additional Editor Comments:

Dear Stefanie Hartmann:

Thank you for submitting your manuscript. You will see that the reviewers are at odds about the manuscript. I would be grateful if you would carefully consider the points raised by the more critical of the reviewers and adjust your manuscript accordingly. If you choose to follow this route, I will send the revised version back to the more critical reviewer and to another reviewer.

I look forward to hearing form you

thanks

Brock Fenton

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

________________________________________

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: I Don't Know

________________________________________

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

________________________________________

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Reviewer #2: Yes

________________________________________

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: 1. English needs a thorough review. Grammatical errors and inelegancies are common and must be fixed.

Thank you for your hint. Our manuscript has now been worked through by a professional native speaking proofreader (Line 1-469).

2. Remove speculations and lengthy details

We changed a few sentences according to your recommendation, we especially hope the English proofreading has helped this point throughout the entire manuscript. If you still feel we need to shorten some aspects, please let us know which exactly and we will do our best.

3. Why no comparison with the results of the British studies?

We list the collisions reported in Minderman et al 2012 and 2015 as well as Moyle et al 2016, both from their systematic searches as well as questionnaires (Line 393-400, line numbers refer to the manuscript with track changes). We compare their conclusions with our results. If we have missed further British studies of relevance to our manuscript, please let us know so we can include those also.

However we include no comparison with respect to avoidance behaviour, the main focus of the mentioned British studies, as this is out of the scope of our study. Our technical approach covers only a 20m-radius around the SWT and we therefore cannot draw any conclusions about a general avoidance of the area around the SWT, which takes place up to several 100m distance according to the British studies.

Reviewer #2: Summary

In this experiment, the authors examined the proximity to which bats approach small wind turbines (SWT) by temporarily installing a turbine at six different sites and employing different operational treatments. I think the key interesting result here is that bats appeared to approach SWT more closely than a pole used at the same site as a reference point, but only when it was not operational at all, or the blades were moving slowly. However, whilst the authors discuss the results of the treatment where the blades were moving slowly, I couldn’t see any arguments for why a turbine and a vertical pole were approached differently if the blades were not moving at all – it would be interesting to consider what kind of behavioural mechanism may result in this finding.

We think that the reason for this finding could be that the telescope pole is not such an conspicuos new structure as an SWT with three rotor blades is. Like, it very much resemble a thin dead tree and does not take in as much room as the blades. We now include this possible explication in the manuscript (L 366-369).

Whilst I felt there were some interesting results in this study, I have a number of concerns, primarily:

• The relevance of the study to realistic installation scenarios. Were the installations following any guidelines? From the photos provided some of these locations did not look very suitable for turbines (in terms of laminar flow and maximal wind speed), and are not in accordance with the Eurobats guidance on locating small wind turbines.

This is correct. We explicitly chose to simulate a worst-case scenario and therefore placed the SWT closer to the vegetation than recommended in the guidelines or economically reasonable. While we mention that we aimed at a worst-case scenario, we had not emphasized this aspect so far. It is now included in the manuscript both in the methods (L. 104, L. 133-135) as well as in the discussion (L. 411-413).

• The wording of the aims are very vague (L82 – 89) – it needs to be clearer what is actually being measured that will enable the aim to be addressed. What does damage mean? Also, I think the bats’ response is going to vary so much by scenario there's no way of knowing whether this is max damage or not.

You are right. We now specifiy that we are interested in the amount of exploratory flights, rotor passes and subsequent collisions. We also changed our wording so that it becomes clearer we only aim to approach the maximum damage but of course we cannot claim to have reached this aim (L 101-111).

• Results felt quite fragmented in places and lacking key information – findings from a linear model are presented but with no test statistics and with the results spread over a large area e.g. the headline result is given on one page but we don’t find out until the following page that the ability of the model to explain the variation in the dataset is tiny (R2 of 1%).

We now present the findings more concisely and included a table with the test statistics (Table 1, L. 290-297)

• The final conclusion (L28-30) of the abstract is that casualties can reach relevant levels over time. I do not feel the authors have the data to make this claim, and no information is presented on what constitutes a “relevant level”. Bats were monitored behaviour over 5 days in areas of purposefully high activity – we don’t know what would have happened if the turbines had been in place for longer.

You are right. We now adjusted the wording for a more precise statement and to better match our conclusions (L. 34-41).

Ethics

I found it very surprising that the institute the researchers are based at does not have a process for reviewing potentially harmful research activities. The fact that a bat was killed as a result of this work demonstrates this – this isn’t really a criticism of the nature of the research as there’s clearly a role for these sorts of experiments. However, there is certainly a conversation to be had about what is acceptable risk and what might be considered unethical. Readers may take a variety of views so some consideration in the discussion (or an expansion on the existing text), over and above “it was legal” is warranted.

We had intense communications with the Regional Council, which in our case is responsible for reviewing potentially harmful research activities. We first had tried to prevent bats from entering the rotor with nets, however this did not lead to realistic results as bats were able to cross the rotor unharmed (and we would not have been able to differentiate from real collisions) and also bats approached the rotor from all sites and in 13 out of 14 flights avoided the net in last second (which left us unknowing whether they avoided the rotor or the net). We thus finally agreed on an experimental setup accepting a small number of killed bats (1 per site, at first collision the experiment was to be stopped immediately). We knew- in case of higher collision rates- that this approach would not let us properly calculate the number of collisions per day/week, but that was the compromise to not harm the local populations any further. We supplemented the respective information with the biological component (additonal to the legal part):

„In case of a collision, the SWT was immediately halted and the experiment stopped ahead of schedule at the respective site. Thus, we ran a risk of maximum six bat casualties in total across all sites, for which we had obtained a permit of the Regional Council Freiburg (05.04.2018, file reference: 55-8852.44/100) to ensure our experiment complies with the federal law on nature protection of the European Union and to avoid negative effects on population level.“

If you consider it helpful to include further details or a repetition of the above aspects in a different part of the manuscript, please let us know.

General

Detailed comments

L24: The term reference pole is introduced here with no context – a little rephrasing here would clarify the purpose of the pole.

You are right, we now formulate it more generally in the abstract and provide more detail about the reference pole in the manuscript itself (L128-132).

L31: The last line of the abstract felt a bit abrupt – a little rephrasing/expansion/clarification is needed.

Thanks for your comment, we now reworded the last two sentences, see above.

L67: the authors consider that a major shortcoming of previous research is that the turbines had been up for longer time periods than in this study, meaning that the immediate effects could not be addressed. I agree that it’s really interesting to look at whether effects change over time but feel that this is only a shortcoming if your main interests are quantifying short term effects. If you’re interested in what happens over the longer term, this is not a short coming at all. I would rephrase to reflect this.

Thank you for the suggestion, point adopted (L81-84).

L74: is there a ref to back up the statement that the “preferred installation [of SWT] in close

proximity to the buildings which they supply with energy”? Suitable locations for SWT need to avoid areas where barriers (such as buildings, treelines) can disrupt wind flow. Indeed this is why turbines are no longer installed in urban areas because the pattern of wind flow is not suitable. Most of the SWT I am aware of today are installed in fields, away from buildings.

In Germany we find most SWT closer than 150 m to the respective buildings. All 10 SWTs in our first study (reference 25) had less than 100 m distance. However as this appears to be a regional or national finding, we therefore decided to rephrase our sentence to a more general comparison with large wind turbines (L91-96).

L82: not sure “unravel” is the most appropriate word to use here – quantify/characterise?

We are sorry for our apparently improvable english! We now have sent the manuscript to proofreading prior to resubmission. In this specific case, the wording has been changed already in response to a prior comment.

L85: change acoustical to acoustic

Thank you, point adopted.

L92: I am not familiar with the term “transfer structures” (and at L177 “transfer activity”) – some clarification or rephrasing needed. I presume this means commuting to foraging sites?

Yes, the term is now specified (L 115-117). If „commuting structure“ or „commuting activity“ is the official term, we can also substitute „transfer“ throughout the entire manuscript?

L100: We need more information on the expt set up e.g. distance to buildings/vegetation etc. How far away was the reference pole to the turbine?

The reference pole was present only during the first night, right at the same position where the SWT was to be standing during the following nights. It served as a reference for distance measurements and to allow acoustic recording in the same height (4 m) as with SWT. We see this point has not been adequately described in the first version of the manuscript and now improved it, as well as providing additional information on the distance to vegetation etc (L128-136).

L128: We need some information on what these acoustic data were used for? I was expecting to see an analysis of activity at the turbine vs. ref pole, but I think it was to obtain species id for some of the trajectories?

Yes, point adopted (L. 165).

L138: We need information here on the metrics being quantified e.g. minimum distance

Point adopted (L. 188-196).

L160: It would be helpful if the stats could refer back to the specific objectives being addressed and in the same order as the introduction. Is the unit of replication here the flight trajectory? Some discussion that there will inevitably be pseudo-replication in the analyses since it will not be possible to distinguish individuals. Also, in the results there is a comparison of the turbine and the ref pole but there is no explanation here on how or why they are doing this.

With the rephrased introduction, the match between the aims and the presented statistics should now be improved.

Yes, flight trajectory is the unit of replication, we now provide further information on this aspect and also how distance was calculated (L. 188-196).

Concerning pseudoreplication: Yes, you are right, some individuals will have entered the filmable area more than once. However given the very high number of individuals due to the chosen hot-spots, most flightpaths will belong to different individuals. It would also simply not have been feasible to individually assign each flightpaths to individuals because individuals were not marked or identifiable. Also we think individual differences in approaching the SWT are neglible compared to the differences in species or due to hot-spot-type (transfer, hunting or roost). However we now acknowledge this point in the manuscript.

The aspects concerning the reference pole should become clear now with the additional information provided to answer a prior comment.

L164 & 203: these sentences sounds awkward and need rephrasing

Point adopted (L. 213-216).

L194: Was there a reason for picking 10m here as the distance? I realise any distance picked might be arbitrary but if there was a rationale for the 10m it would be useful to clarify here.

It was chosen for a better comparison with a previously conducted study (where it was chosen arbitrarily).

L197: These are the key results but there is much which I found unclear here. Are these - are these modelled estimates or raw data? Where are the results of the model and the test statistics? We find out on the next page that R2 for fixed effects is only 1% - I found this quite hard to reconcile with the confidence intervals shown in Fig 3.

We have closely collaborated with a professional statistical company (oikostat.ch, see also Acknowledgements) who confirmed all our statistical tests and interpretations for correctness.

The confidence interval is mainly determined by the sample size, whereas the R2 is a measure of the effect size in relation to the overall variance in the data. Therefore narrow C.I.s do not contradict the fact that our fixed effects explain only little variation. We have a large sample size (n=7850) compared to the variance. The effect sizes are clear (small uncertainty), but small compared to the variance in the raw data. This means that beside the effects we describe, there are also other factors determining the variance in the data.

L215: the number of bats passing in front of the rotor when it’s not moving doesn’t seem like a very relevant metric?

We think it still indicates the amount of curiosity bats show towards the blades in general. It is no indicator for collision risk, however.

L225: Some of the sites that differed in vegetation structure etc also differed in species composition so I just don’t feel there is sufficient replication to tease this all out

You are right, it is impossible to derive which of the different factors have a major or minor contribution to the site differences. We changed the phrases accordingly (L 298-299).

L265: I’m not familiar with the term “structure bound” for bats? Pipistrellus sp are also known to forage along forest edge and interiors, and are closely associated with buildings.

Point adopted, our proof reader suggested to switch to “structure-oriented” which we did (L.300, L.345).

L279 (and abstract): these point estimates should only be given with the confidence intervals.

Point adopted.

L284: comment is made that no habituation over the 5 days was evident but I think it’s important to note that the treatments changed each night so this doesn’t seem very surprising.

You are right. We could have expected to see a difference only in case it had come to collisions already on day 2 of those SWT which started with full motor speed (compared to those who had moving motor blades on day 3, after a day of habituation). This was our original idea for why we used this experimantal design. But as we had only one, and late (day 5) collision, drawbacks on habituation make little sense in this case. We there fore removed the respective phrase (L. 376).

L294: 64 of 65 trajectories (not bats)

I am sorry, of course you are right. Thanks! As we want to refer to individuals (not trajectories) in our argument, we rephrased accordingly by replacing the specific numbers with „most bats“(L.388).

L347: interestingly, this siting recommendation was also made by Minderman et al. but for opposite reasons (because of the apparent disturbance / avoidance effect detected in that study).

Both mortality and disturbance are negative influences of SWT on local populations, which can be minimized with increasing distances between SWT and bat hot-spots. We hope consistent recommendations across a variety of studies help to implement respective avoidance measures. Thank you for your constructive comments on our manuscript!!

________________________________________

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

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17 May 2021

PONE-D-20-40193R1

Collision risk of bats with small wind turbines: worst-case scenarios near roosts, transfer paths, and hunting structures

PLOS ONE

Dear Dr. Stephanie Hartmann :

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

==============================

ACADEMIC EDITOR:

Thank you for attending to the issues raised by the reviewer.  Note there there are a few other comments for you to consider.

thanks

Brock

==============================

Please submit your revised manuscript by Jul 01 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols.

We look forward to receiving your revised manuscript.

Kind regards,

Brock Fenton

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

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1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

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Reviewer #2: Yes

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Reviewer #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: Thanks for your revisions, which I feel have greatly improved this ms. I do think the term "transfer activity/route" either needs defining earlier on or substituting with "commuting" which I think is more commonly used.

I did have a query about the comparison on the % of flights passing within 10 m of the turbine (ref 25). Using % as a metric is only a fair comparison if both recording set-ups have exactly the same field of view. In the methods you state that bats could be detected upto 20 m from the turbine. Was this the same with the study outlined in ref 25? Clearly, the narrower the field of view the the higher the proportion of flights will be recorded within 10m of the turbine.

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Reviewer #2: No

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8 Jun 2021

Dear Brock Fenton, Dear Reviewer,

thank you very much for your answer!!

Let me first update the Funding and Competing Interest sections, below you will find the answers to the last revision.

Funding:

This project was funded by the German Bundesamt für Naturschutz, FGII 4.3/Naturschutz und erneuerbare Energien, Projekt zum Forschungs- und Entwicklungsvorhaben aus Mitteln des Bundesministeriums für Umwelt, Naturschutz und nukleare Sicherheit (BMU), Einzelplan 16, Kapitel 1604, Titel 54401. Förderkennzeichen: 3517860600

The funder provided support in the form of salaries for authors [S.A.H., S.G, F.G., B.L., H-S.-W, R.B.], but did not have any additional role in the study design, data collection and preparation of the manuscript. The decision to publish internationally was made together with the funder. The additional salary necessary for the preparation of the english manuscript as well as the revisions was covered by FrInaT GmbH.

Competing Interests statement

During the time of the conduction of the study, all authors except Klaus Hochradel have been affiliated with FrInaT GmbH. The project itself was based with FrInaT. Once the project was finished, two authors (S.G., F.G.) were hired by Oekofor GbR. Oekofor and FrInaT collaborate with several projects and have no competing interests in a financial, professional or personal aspect.

The commercial affiliations to FrInaT GmbH and Oekofor GbR do not alter our adherence to PLOS ONE policies on sharing data and materials.

Last revision

I will reply to each point as follows:

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

I double-checked with the literature used in the first submission and could not find any difference in the cited literature. Please let us know if this is a general comment or whether you specifically refer to our reference list- and which specifically you think has been changed.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #2: (No Response)

________________________________________

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #2: Yes

________________________________________

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

________________________________________

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #2: Yes

________________________________________

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #2: Yes

________________________________________

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #2: Thanks for your revisions, which I feel have greatly improved this ms. I do think the term "transfer activity/route" either needs defining earlier on or substituting with "commuting" which I think is more commonly used.

Thank you for answering our question concerning this term. We now changed it to „commuting“ throughout the entire manuscript.

I did have a query about the comparison on the % of flights passing within 10 m of the turbine (ref 25). Using % as a metric is only a fair comparison if both recording set-ups have exactly the same field of view. In the methods you state that bats could be detected upto 20 m from the turbine. Was this the same with the study outlined in ref 25? Clearly, the narrower the field of view the the higher the proportion of flights will be recorded within 10m of the turbine.

Good point and I apologize for overlooking the query in the first revision. Although the technical equipment was identical in both studies, the field of view around the turbine would have been different simply due to the different heigth of the turbine (� the taller SWT turbine was a few meters further away from the camera simply because of the height, even though the aperture angle of the camera was identical). Thus the field around the heigher turbine in the reference was wider. As the field of views are different, percentage as a metrics is not a good comparison, i agree. We now remove the direct comparison of flight paths percentage/numbers in relation to turbine distance between both experiments and focus more on the overall difference in bat activity, exploratory behavior and rotor passes (L. 284-290).

________________________________________

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #2: No

Additonal changes

I detected a grammatical error in Line 294 and changed it (has-->have).

14 Jun 2021

Collision risk of bats with small wind turbines: worst-case scenarios near roosts, commuting and hunting structures

PONE-D-20-40193R2

Dear Dr.  Stefanie Andrea Harmtman

Thank you for carefully attending to and addressing the reviewer's comments.   Nicely done.

Brock Fenton

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Brock Fenton

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

17 Jun 2021

PONE-D-20-40193R2

Collision risk of bats with small wind turbines: worst-case scenarios near roosts, commuting and hunting structures

Dear Dr. Hartmann:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

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on behalf of

Dr. Brock Fenton

Academic Editor

PLOS ONE