Notes on Winter Bat Mortality, Hibernation Preferences, and the Demographic Structure of Deceased Individuals from One of Europe’s Largest Bat Colonies

Understanding the drivers of population dynamics in long-lived, slow reproducing species such as bats is critical for conservation, particularly during vulnerable life history stages like hibernation. We reviewed winter mortality records from more than 109 monitored hibernacula in Bulgaria. We found that unusual mortality events (UME > 7 individuals) were recorded at only four sites, involving carcasses from at least five species, indicating that such events are uncommon and likely under-detected due to uneven survey effort. Because the conditions under which bats hibernate can shape vulnerability to stressors, we used available long-term survey data to outline species-specific hibernation preferences as a first step toward identifying ecological settings that may influence winter mortality risk. Across Bulgaria, species exhibited distinct preferences for hibernacula: most bats selected humid, low-altitude caves, whereas others occupied colder, high-elevation roosts. Roost temperatures and altitudes differed significantly among species, with Miniopterus schreibersii using the broadest range of conditions, yet neither variable explained variation in colony size. We further analysed the age structure of deceased Miniopterus schreibersii (Bonaparte, 1837) from Bulgaria’s largest hibernation colony following mortality events in winter 2022. Carcasses spanned a wide range of age classes, yet younger individuals predominated, consistent with the idea that early-life mortality represents a key demographic filter in bats. These findings emphasise the need for consistent mortality monitoring in bats, using standardised protocols that account for detection biases, scavenger removal, and site-specific variation. Such efforts are essential for clarifying the roles of environmental extremes, disease, and human disturbance in winter mortality.