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Article

Main Causes of Bat Mortality Obtained Through Admission to Rescue Centres

by
Alfonso Balmori-de la Puente
1,* and
Alfonso Balmori
2,*
1
Zoonotic Diseases and One Health Group, Biomedical Research Institute of Salamanca (IBSAL), Centre for Environmental Studies and Rural Dynamization (CEADIR), University of Salamanca, 37007 Salamanca, Spain
2
Environment Department of Castilla y León, 47014 Valladolid, Spain
*
Authors to whom correspondence should be addressed.
Diversity 2025, 17(8), 567; https://doi.org/10.3390/d17080567
Submission received: 20 July 2025 / Revised: 9 August 2025 / Accepted: 11 August 2025 / Published: 12 August 2025
(This article belongs to the Section Animal Diversity)
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Abstract

Bats have decreasing population trends around the world, and knowledge on the causes for this decline is the first step to improving conservation and management strategies to restore their populations. An important source of data for the study of the causes of bat mortality is the admissions to rescue centres. The aim of this work was to identify the different causes of bat admissions to rescue centres in Castilla y León (Spain) over more than 30 years, analyzing the importance of the threats for different species, as well as the tendency of anthropogenic causes over the years, such as the increase in industrial wind-power facilities. The dataset included 791 bats (568 dead and 223 injured). The species with the largest number of entries was Pipistrellus pipistrellus at 451, followed by 82 Hypsugo savii, 64 Plecotus sp., 63 Tadarida teniotis, 42 Eptesicus serotinus and 24 Nyctalus lasiopterus. The most important known causes of entry for these selected species were collisions with wind turbines (n = 160); immaturity-related causes (e.g., orphan individuals with insufficient foraging and flight skills prone to injury or starvation) (n = 93); weakness due to starvation (n = 75); trauma: blow of unknown origin (n = 69); shooting: vandalism with a gun (n = 15); carnivore bite: predation (n = 8); road kill: road accident (n = 3); and disease: sickness (n = 3). In addition, there were many admissions which lacked a known cause (n = 294). The species with the most carcasses for collisions with wind turbines was Pipistrellus pipistrellus at 100, followed by 40 Hypsugo savii and 15 Nyctalus lasiopterus. As expected, the number of bats that collided with wind turbines showed a significant temporal correlation with the number of wind farms deployed in the territory, and they mainly occurred in September and October, as has also been found in other studies.

1. Introduction

Bats have decreasing population trends around the world [1,2,3,4,5,6], and information on the causes for this decline is the first step to improve conservation and management strategies and propose actions to restore their populations. However, due to the peculiar characteristics of bats, their nocturnal, elusive habits and dearth of research on natural populations, this information is quite limited [6].
Many studies on bats focus on a single cause, such as mortality on wind farms [7,8,9,10,11,12], diseases [4], white-nose syndrome [13,14,15], predation [4,16,17], accidents and trauma [4,16], contaminants and insecticides [18], or habitat loss and roost disturbance [1]. However, there are few studies that review in depth the broad set of causes of bat mortality [16,19].
Admissions of animals in rescue centres are an important source of data for the study of the causes of mortality in animals [20,21,22,23], particularly for birds and some mammal species [24,25,26,27], but have been used only recently for bats [4,17,28,29,30]. This information has high research potential and can improve knowledge on animal biology, new anthropogenic impacts and species conservation [20,27], and therefore serves as an approximation of the anthropogenic impact on wild populations, which can help propose measures for species conservation and education [20,22,23]. However, such databases have been barely used [20].
The aim of this work was to identify the different causes of bat admissions to rescue centres in Castilla y León (Spain) for more than 30 years. This study tries to determine the most important causes for different species, as well as temporal fluctuations both over the study period and seasonally. These issues are discussed according to anthropogenic impacts in the landscape, which may be causing increasing mortality in some species. We hypothesize that (i) the number of admissions per species should vary according to species behaviour and (ii) anthropogenic impact derived from renewable energy deployment (e.g., wind farms) has become more important in recent years, (iii) especially affecting species which perform some migratory movement.

2. Methods

Castilla y León (Spain) is the largest region (administrative unit) in Europe, with a surface area of 94,222 km2. The centre of the region is a plateau that is surrounded by mountains. The network of wildlife rescue centres in the region receives injured or dead animals collected by private individuals or environmental agents (for more information see [26]). Specifically for wind-power facilities, dead animals are collected by the companies and by environmental agents who brought them to the rescue centres of the environmental department. Once the animals have reached the centre, a specialist performs morphological identification of the species, and a veterinary pathologist thoroughly examines the specimens, performs a necropsy (when possible) and tries to determine the main cause of injury or death. Some causes of death are sometimes motivated by a combination of factors, but the qualified expert assigns it to the category which fits better with the results of the analysis, which does not mean that other causes do not interplay. In some cases, there may have been a delay in carcass discovery, since carcass condition varies by season and disposition on the landscape. The date of admission to the rehabilitation centres was in most cases in the days after the location in the field.
First, we analyzed >30 years (1992–2023) of data on bat admissions to rehabilitation centres in Castilla y León. For the analysis, we selected the six species with the greatest number of admissions and the most significant causes (the causes of entry observed in at least two different species). The data were analyzed using R software, version 4.2.3 [31]. Descriptive analysis of the cases of admission was carried out to determine absolute frequencies per cause of admission and their relative frequency in each species. Fisher’s exact test (recommended for datasets with small expected frequencies for each combination cell) was performed to study potential associations between the causes of admission in all species. The pairwise Fisher test function in R together with the Bonferroni correction for multiple comparisons was used in every species pair. The significance level was established at p < 0.05.
Second, we investigated the bats that were killed by wind turbines and admitted to the rescue centres in Castilla y León. All the specimens collected were found dead under wind turbines after colliding with them or due to barotrauma. Information on the deployment of wind energy and the installed power in Castilla y León has been compiled [32]. At the beginning of the twenty-first century, wind farms were mainly installed in the mountainous areas but recently have also been installed on the plains in the centre of the region. Therefore, we investigated whether there are fluctuations in seasonal mortality and fit a linear model to study whether there is a temporal correlation between the accumulated installed wind power and annual bat mortality from this cause. In this case we restricted the analysis to the period including 1992–2022 due to incomplete information in 2023.

3. Results

The full dataset included a total of 791 bats (568 dead and 223 injured) that were admitted in the rehabilitation centres due to several causes of admission during the study period (Table 1). The six species with the highest number of occurrences (>90% of all entries) were Pipistrellus pipistrellus (451), Hypsugo savii (82), Plecotus sp. (64), Tadarida teniotis (63), Eptesicus serotinus (42) and Nyctalus lasiopterus (24) (Table 1).
The most common causes of entry for these selected species (>90% of all entries; Table 1) were collisions with wind turbines (n = 160, all dead) followed by immature condition (n = 93), weakness (n = 75), trauma (n = 69), shooting (n = 15), carnivore bite (n = 8), road kill (n = 3), disease (n = 3) and a large number of cases with unknown causes (n = 294).
Among the bat species considered in this study, Pipistrellus pipistrellus prevails across all causes of admission, with most carcasses found due to collisions with wind turbines, immature condition, trauma and cachexia, while Hypsugo savii was just affected by collisions with wind turbines among known causes. For Plecotus sp. the main known cause of admissions to the rehabilitation centres was immature condition; for Tadarida teniotis it was cachexia and trauma of unknown origin; for Eptesicus serotinus, the three mentioned before, and Nyctalus lasiopterus, it was collisions with wind turbines (Table 1 and Figure 1).
We observed associations between all selected species as a whole and causes of admission (Fisher’s exact test p-value = 0.0005). After testing associations between pairs of species and causes of admission, Hypsugo savii and Nyctalus lasiopterus were involved in accidents with wind turbines with higher frequencies than expected in comparison with other species and other causes (p < 0.05 in pairwise Fisher test corrected for multiple comparisons) (Figure 1). In addition, weakness was an important threat for Tadarida teniotis (p < 0.05 in pairwise Fisher test corrected for multiple comparisons) (Figure 1). Finally, other apparently “less-important” associations were found (see Supplementary Table S1 with the full set of comparisons).
The temporal changes for the main known causes of admission for the selected species show an increase in the number of admissions in the rescue centres over the years (Figure 2). In addition, collision with wind turbines appeared as an important threat, especially in recent years. If we assess the monthly mortality on wind farms for the selected bat species with the highest number of occurrences over the study period, we find that cases varied seasonally, and the months with the largest number of accidents with wind turbines were September and October (Figure 3).
As of 2022, the administrative division of Castilla y León had 6624 MW of wind energy [32] of Spain’s total 29,994 MW [33]. Figure 4A shows the accumulated installed power of wind farms and annual bat mortality from collisions with wind turbines (all species considered). Collisions with wind turbines occurred most frequently in Pipistrellus pipistrellus (n = 100), Hypsugo savii (n = 40) and Nyctalus lasiopterus (n = 15). There was an increase in the number of carcasses found as installed power increased over the years (R2 = 0.66 (p < 0.001)) (Figure 4B).

4. Discussion

Information from animal rehabilitation centres has been used in other studies mainly to determine the causes of bird and bat mortality [4,17,24,25,29], and to collect information and occurrences for different species [34]. This study extends its application to compare the causes of entry for different bat species on a temporal scale.
Out of the six species with the greatest number of entries into the rehabilitation centres, three of them (Pipistrellus pipistrellus, Hypsugo savii and Nyctalus lasiopterus) had collisions with wind farms as the main known cause of admission, which were associated with a fatality rate of 100% (Table 1 and Figure 1). The species found in this work with the highest collision occurrences with wind turbines are the most abundant [35], such as Pipistrellus pipistrellus, those inhabiting mountain areas where more wind turbines are mounted such as Hypsugo savii, or migratory species such as Nyctalus lasiopterus, which is consistent with what has been observed in other studies in southern Iberia [12].
The increase in the number of bat admissions over the years found in this study (both due to general causes and specifically caused by wind turbines) (Figure 2) may be partly explained by a recent greater awareness of people, as well as the monitoring that is carried out for the injured species specially in wind farms. The months in which most accidents with wind turbines occurred were September and October, close to what has been observed in [12], which may be attributed to bat movements (Figure 3). Precisely, in temperate latitudes most deaths of bats on wind farms occur during late summer and autumn [9,11,19,36,37]. In the USA, collision mortality is greater for migratory tree-roosting species [9]. As expected, the number of bats that collided with wind turbines showed a significant temporal correlation with the number of wind farms deployed in the territory (Figure 4). For some species, deaths at wind turbines far exceeded other known sources of mortality, and in North America, collisions with wind turbines and white-nose syndrome are the main causes of mortality in bats [19].
It may be surprising that cat predation is absent from the data, as cats are a significant cause of bat mortality in urbanized areas of some countries. For instance, in Italy, cats are the leading cause of adult bat admittance to recovery centres [28]. However, in a review of the Red Book of Mammals of Spain, cats are not even mentioned as a threat to endangered species [38]. In the seven species of bats that have a record published in the virtual encyclopedia of Spanish vertebrates, cats do not appear as predators in any of them, but the owl, the long-eared owl, various diurnal birds of prey and the genet do appear [39,40,41,42,43,44,45]. The only published case in Spain of predation by a domestic cat occurred in a breeding colony of Myotis myotis [46]. Therefore, it seems that in Spain cat predation is a very localized and unimportant problem. However, it is possible that some bats killed by carnivores were killed specifically by cats, and we cannot rule out that some bats killed by cats or other carnivores have been included in “unknown” cause, due to the absence of clear signs upon arrival at the recovery centre.
Unfortunately, the variable degree of sampling effort for the different causes of admission impedes an accurate comparison for assessing their relative contributions. The most general common factor (except in T. teniotis and N. lasiopterus) for admission for the six main species selected in this study was “unknown”. Vezyrakis et al. [23] also found that unknown causes represent approximately one third of all admissions. This could be explained by highly damaged specimens or new threats that could not be identified either by the collector or veterinary personnel. In fact, monitoring programmes of bat mortality do not occur evenly for all human facilities; for instance, they are lacking in new roads under construction. In this regard, it is important to point out the potential sources of mortality that are underestimated in this study (e.g., roads are expected to generate huge amounts of bat mortality: [47,48]). This difference could bias mortality estimates from a conservation point of view. The roads cover enormous amounts of landscape and might well be the first cause of mortality for many bat species. If conservation practices continue with this trend, bat species susceptible to road collisions will continue declining with almost no detection and interest from conservation stakeholders. Therefore, it is important to point out that the data collected in the rescue centres contains only a fraction of the cases that exist in the wild. Data drawn from rehabilitation facilities cannot replace fatality monitoring data, as the latter can help in correcting potential uncertainties and biases extracted from the former. However, it also provides information of interest for species whose causes of mortality are barely known.
These data may be especially relevant for animals that live in close proximity to people because of increased odds for detection and admission to the rehabilitation centres [17]. Except for the case of wind turbines, these data were collected by passive sampling and do not necessarily represent a random sample of individuals in the environment. It must be noted also that many injured animals are never found in the wild, and common species and immature individuals usually dominate these admissions [23].
Understanding the increases in mortality from anthropogenic sources and potential drivers of population declines would contribute to the recognition of human impacts on bat populations, identification of mitigation initiatives, and testing of the efficacy of solutions to stabilize or improve population trends [6,17,19,29]. Bats could benefit from policy, education and conservation actions with the objective of counteracting human-caused mortality [19,22], and technical documents could help with the adoption of measures to avoid injuries to the bats on wind farms [49]. However, identifying threats and good practices and producing technical documents can have no effect on the application of conservation practices if the former are not supported by the national legislation [50].

5. Conclusions

This study demonstrates that wind turbine collisions represent the leading cause of bat mortality among the six most frequently admitted species in wildlife rehabilitation centres in Castilla y León, with Pipistrellus pipistrellus, Hypsugo savii, and Nyctalus lasiopterus being the most affected. Notably, these three species exhibit specific ecological traits—such as high abundance, mountain habitat use, or long-distance migration—that may increase their vulnerability to wind energy infrastructure. The observed 100% fatality rate associated with turbine collisions and the significant increase in mortality concurrent with rising wind-power capacity underscore the urgent need for targeted mitigation measures.
Additionally, seasonal trends indicate that mortality peaks in September and October, coinciding with post-breeding dispersal and migratory movements, a pattern consistent with findings from other regions in temperate latitudes. This temporal pattern reinforces the importance of integrating seasonal risk assessments into wind farm management and planning.
Species-specific associations with other causes of admission, such as weakness in Tadarida teniotis or immature condition in Plecotus spp., further illustrate the need for tailored conservation responses. However, the high proportion of cases with undetermined causes and the passive nature of data collection limit the extrapolation of results to broader population-level impacts. These limitations highlight the importance of complementing rehabilitation centre data with systematic field-based mortality monitoring, especially around roads and other under-surveyed infrastructure.
Despite these constraints, data from rehabilitation centres remain an important complementary tool for detecting mortality drivers in bats, especially for species that are poorly monitored in the wild. To effectively address the anthropogenic threats identified in this study, conservation strategies must move beyond technical recommendations and be supported by robust legal frameworks, public engagement, and long-term ecological monitoring. Only through an integrated approach can we mitigate the growing impact of human infrastructure on bat populations and support their conservation in rapidly changing landscapes.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/d17080567/s1, Table S1: Significance results of pairwise Fisher Test comparisons after applying Bonferroni correction for multiple comparisons between each species pair (ns, not significant; *, p < 0.05; **, p < 0.01; ***, p < 0.001; ****, p < 0.0001).

Author Contributions

Conceptualization, A.B.-d.l.P. and A.B.; methodology, A.B.-d.l.P. and A.B.; validation, A.B.-d.l.P. and A.B.; formal analysis, A.B.-d.l.P.; investigation, A.B.-d.l.P. and A.B.; resources, A.B.; data curation, A.B.-d.l.P. and A.B.; writing—original draft, A.B.-d.l.P. and A.B.; writing—review and editing, A.B.-d.l.P. and A.B.; supervision, A.B.-d.l.P. and A.B. All authors have read and agreed to the published version of the manuscript.

Funding

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data Availability Statement

The datasets generated and analyzed during the current study are available from the organization “Junta de Castilla y León” through a formal request.

Acknowledgments

“Junta de Castilla y León” provided the information for this work through a formal request. This study has not had any funding support.

Conflicts of Interest

The authors do not have relevant financial or non-financial interests to disclose.

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Figure 1. Relative contribution from selected species to the causes of admission with the highest numbers of entries to rehabilitation centres.
Figure 1. Relative contribution from selected species to the causes of admission with the highest numbers of entries to rehabilitation centres.
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Figure 2. Temporal changes in known causes of bat admissions to rehabilitation centres for selected species, measured as the relative contribution of wind farm accidents to the total over the study period.
Figure 2. Temporal changes in known causes of bat admissions to rehabilitation centres for selected species, measured as the relative contribution of wind farm accidents to the total over the study period.
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Figure 3. Seasonality of bat mortality on wind farms for selected species. Species–colour legend used as in Figure 1. Grey bars in the background indicate total counts per month.
Figure 3. Seasonality of bat mortality on wind farms for selected species. Species–colour legend used as in Figure 1. Grey bars in the background indicate total counts per month.
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Figure 4. (A) Installed power (MW) in the region (light blue—annual; blue—cumulative) and annual bat mortality on wind farms over the study period (green). (B) Linear regression between the cumulative installed power (MW) and bat mortality (log 10 + 1) over the study period. The fitted model includes the fitted model equation, p-value, R2 and 0.95 confidence interval (gray area). Note that all species registered in this study and not just the six more abundant ones are included in this figure. The installed wind energy data was extracted from Junta de Castilla y León, 2022.
Figure 4. (A) Installed power (MW) in the region (light blue—annual; blue—cumulative) and annual bat mortality on wind farms over the study period (green). (B) Linear regression between the cumulative installed power (MW) and bat mortality (log 10 + 1) over the study period. The fitted model includes the fitted model equation, p-value, R2 and 0.95 confidence interval (gray area). Note that all species registered in this study and not just the six more abundant ones are included in this figure. The installed wind energy data was extracted from Junta de Castilla y León, 2022.
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Table 1. Bats admitted in the rehabilitation centres during the study period and their causes of admission. Selected bat species and causes are highlighted in light green (see Figure 1). Note that cryptic bat species or those with very few reported cases in the study area could not be identified in this work, and taxonomy for some bat species may have been resolved after data collection. Wind turbine: death in a wind farm; immaturity-related causes (e.g., orphan individuals with insufficient foraging and flight skills prone to injury or starvation); cachexia: weakness due to starvation; trauma: blow of unknown origin; shooting: vandalism with a gun; carnivore bite: predation; captivity: coming from captivity; road kills: road accident; disease: sick specimen; snare: trap or snare; electrocution: electrocution in electrical lines or pylon transformer; unknown: absolutely unknown cause.
Table 1. Bats admitted in the rehabilitation centres during the study period and their causes of admission. Selected bat species and causes are highlighted in light green (see Figure 1). Note that cryptic bat species or those with very few reported cases in the study area could not be identified in this work, and taxonomy for some bat species may have been resolved after data collection. Wind turbine: death in a wind farm; immaturity-related causes (e.g., orphan individuals with insufficient foraging and flight skills prone to injury or starvation); cachexia: weakness due to starvation; trauma: blow of unknown origin; shooting: vandalism with a gun; carnivore bite: predation; captivity: coming from captivity; road kills: road accident; disease: sick specimen; snare: trap or snare; electrocution: electrocution in electrical lines or pylon transformer; unknown: absolutely unknown cause.
SpeciesConditionWind TurbineImmatureCachexiaTraumaShootingCarnivore BiteCaptivityRoad KillDiseaseSnareElectrocutionUnknownTotal
Pipistrellus pipistrellusDeath1004813401612111125339
Pipistrellus kuhlii1000000000001020
Pipistrellus pygmaeus3000000000058
Hypsugo savii4000000000004282
Plecotus sp.0211100010028
Tadarida teniotis314911000002039
Eptesicus serotinus25150001000418
Nyctalus lasiopterus150010000000824
Nyctalus leisleri80000000000311
Nyctalus noctula3000000000069
Myotis myotis2000000000002
Myotis nattereri1000000000023
Myotis daubentonii0000000000 011
Miniopterus schreibersii2000000000002
Rhinolophus ferrumequinum0001000000012
Pipistrellus pipistrellusInjured027309813000034112
Pipistrellus kuhlii0000000000000
Pipistrellus pygmaeus0000000000011
Hypsugo savii0000000000000
Plecotus sp.060000000005056
Tadarida teniotis001912000000224
Eptesicus serotinus04732000100724
Nyctalus lasiopterus0000000000000
Nyctalus leisleri0000000000000
Nyctalus noctula0000000000000
Myotis myotis0010100000013
Myotis nattereri0000000000011
Myotis daubentonii0010000000012
Miniopterus schreibersii0000000000000
Rhinolophus ferrumequinum0000000000000
Total 18993777016843311326791
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Balmori-de la Puente, A.; Balmori, A. Main Causes of Bat Mortality Obtained Through Admission to Rescue Centres. Diversity 2025, 17, 567. https://doi.org/10.3390/d17080567

AMA Style

Balmori-de la Puente A, Balmori A. Main Causes of Bat Mortality Obtained Through Admission to Rescue Centres. Diversity. 2025; 17(8):567. https://doi.org/10.3390/d17080567

Chicago/Turabian Style

Balmori-de la Puente, Alfonso, and Alfonso Balmori. 2025. "Main Causes of Bat Mortality Obtained Through Admission to Rescue Centres" Diversity 17, no. 8: 567. https://doi.org/10.3390/d17080567

APA Style

Balmori-de la Puente, A., & Balmori, A. (2025). Main Causes of Bat Mortality Obtained Through Admission to Rescue Centres. Diversity, 17(8), 567. https://doi.org/10.3390/d17080567

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