Seasonal and spatial variations in short-distance migrating bats in relation to weather conditions : a study of Eptesicus nilssonii and Pipistrellus pygmaeus

Sammanfattning

In order to investigate the use of remote islands by short distance migratory bats in the Baltic Sea, acoustic bat monitoring was carried out in the Stockholm and Öregrund archipelago from June until November 2024 (Stockholm archipelago) from May until September 2025 (Öregrund) and May until October 2025 (Stockholm). Twenty islands within the study areas were selected for sampling, and one automatic recording unit was deployed at each site during the study period. The surveyed islands were selected to be as distant from the mainland as possible, while remaining accessible for researchers. The hypothesis was that the archipelago environments in the Baltic Sea might be of importance for bats as foraging sites, especially during the post colony period, and that spatial and weather-related variations such as distance to larger land areas from the surveyed islands, as well as unfavorable weather (strong winds, precipitation and lower temperature) could influence bat activity in these environments. Results showed that wind speed and precipitation was the most important weather parameters negatively affecting bat activity during the study period. P. pygmaeus showed a decrease in bat activity of 14 % for every 1 m/s increase in wind speed. Bat activity in E. nilssonii declined with 17 %. Similarly, activity declined with 56 % with every 1 mm/hour increase in precipitation for E. nilssoni, and with 53 % for P. pygmaeus. In addition, bat activity decreased significantly with increasing distance to larger land areas (11.6 % for every 1 km increase), suggesting that more isolated islands are visited less frequent than islands closer to land. These findings suggest that the presence of short distance bats on offshore islands in the Baltic Sea during the post colony period primarily reflects foraging behaviour rather than long distance migration. The study provides insight into the use of coastal environments as important foraging sites for bats, how their activity responds to different weather conditions as well as the locations of islands as foraging sites. These findings contribute to a better understanding of bat ecology during the post colony period in northern archipelago environments.