Bulgaria has a very rich bat fauna and large colonies of bats can be found in caves, mines and other underground roosts. Respectively, there are more than 107 underground roosts that are listed as important bat sites, most of which are protected by statutory laws and are of national or international importance. Despite the existence of formal protection, many roosts face anthropogenic disturbances due to the popularity of outdoor activities, such as caving and the lack of actual regulation. Currently, the evaluation was only based on the size of the colony and the presence of protected species. However, this approach is limited to roosts that contain high diversity and neglects the ones that contain high biotic importance that are highly threatened by various threats. Here, we evaluated conservation priorities and identified the most vulnerable underground bat roosts in Bulgaria, using the Bat Cave Vulnerability Index and proposed measures to adequately protect sites. We found that 32% of the Bulgarian bat roosts assessed are at a "high priority" level for conservation and protection, while 39% are at a "medium priority" that may require constant monitoring. This novel and integrative approach applied to bat roost prioritisation in the country enabled the detection of sites that need urgent conservation attention and is the first step in establishing better strategies for the bat monitoring network in Bulgaria.

Little is known about the winter bat activity in Bulgaria, which poses challenges in monitoring potential deviations in their behaviour as a consequence of the warming climate. Using passive acoustic monitoring, we investigated the winter activity in some of Europe’s largest hibernacula. Our findings reveal cave and species-specific activity patterns. Activity was observed throughout each month of the survey, with distinct peaks on specific days. At one high-elevation site, bat activity was restricted to a single night, while the highest overall activity occurred at the highest elevation site (1325 m). The most active species was Myotis capaccinii (Bonaparte, 1837). While bats were mostly active right after sunset following their usual circadian rhythm, some daytime activity was also observed, including emergence at temperatures as low as -8°C. At sites with sufficient activity data, external temperature emerged as a significant positive predictor of bat activity, with higher temperatures associated with increased activity. Our data also suggest that bats rarely forage near the roost entrances. The observed variability in activity levels between study sites highlights the need for high-resolution, site-specific data rather than broad generalisations.

Urbanisation presents both challenges and opportunities for bat populations. Understanding the composition and distribution of urban bat species is the first step toward their effective conservation. However, comprehensive studies on urban bat diversity remain scarce in Bulgaria. To address this gap, we conducted a year-long acoustic survey on five different habitats in the urbanised territory of Veliko Tarnovo. Our study identified the presence of eight bat species and other distinct acoustic groups within the urban environment. Bats were found to utilise various habitats across the urban landscape. Although the activity was highest during May and September, bats remained active all year round, including the winter. Our study provides insights into synanthropic bats’ seasonal and microhabitat-dependent use of urban areas. These findings will establish a baseline for future research focusing on habitat preferences, population dynamics, and interactions with urbanisation. Such research is essential for the effective management and conservation of bat populations in urban environments.

Elevation is a strong proxy for the thermal environment because it causes a predictable drop in temperature and food availability. This restricts cave-dwelling bats to species with specific metabolic traits, such as torpor or migration to avoid cold stress. In this context, we aimed to reveal how thermal niche differentiation structures 25 cave-dwelling bat assemblages along elevation gradients in two of the largest Bulgarian mountains—Stara Planina and Rhodopi. Multivariate PERMANOVA showed significant differences in bat assemblages among elevation groups (F = 1.616, p = 0.046), with altitude and temperature explaining 32.4% of the variance (p = 0.001). A high degree of species turnover (91.12% dissimilarity), driven by temperature niches, was observed: mesophilic Rhinolophus species dominated warm, low-elevation caves, while cold-adapted Myotis species were more common at high elevations. SIMPER analysis identified R. euryale as an indicator in low-elevation caves (p = 0.012) and the M. myotis/blythii complex at high elevations (p = 0.003). Alpha diversity showed no variation across elevation groups (p = 0.293), indicating that species turnover occurs without overall changes to local diversity. Mid-elevation assemblages lacked specific indicator species and resembled high-elevation communities, forming an ecotone. Thermal niche partitioning, as a physiological filter, shapes cave-dwelling bat assemblages and affects climate change range-shift predictions.

Little is known about the winter bat activity in Bulgaria, which poses challenges in monitoring potential deviations in their behaviour as a consequence of the warming climate. Using passive acoustic monitoring we investigated the winter activity in some of Europe’s largest hibernacula. Our findings reveal cave and species-specific activity patterns. Activity was observed throughout each month of the survey, with distinct peaks on specific days. At one high-elevation site, bat activity was restricted to a single night, while the highest overall activity occurred at the highest elevation site (1325 m). The most active species was Myotis capaccinii (Bonaparte, 1837). While bats were mostly active right after sunset following their usual circadian rhythm, some daytime activity was also observed, including emergence at temperatures as low as -8°C. At sites with sufficient activity data, external temperature emerged as a significant positive predictor of bat activity, with higher temperatures associated with increased activity. Our data also suggest that bats rarely forage near the roost entrances. The observed variability in activity levels between study sites highlights the need for high-resolution, site-specific data rather than broad generalisations.