Notes on the Winter Bat Mortality in Bulgaria and the Demographic Structure of Deceased Individuals from One of Europe’s Largest Bat Colonies

Understanding the drivers of population dynamics in long-lived, K-selected 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 significant die-offs (>30 individuals from at least three species) were recorded at only three sites, suggesting that such events are rare but potentially consequential for local populations. To reveal how habitat use shapes vulnerability during winter, we investigated hibernation preferences of cave-dwelling bats in Bulgaria. 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.