Evidence of the migratory bat, Pipistrellus nathusii, aggregating to the coastlines in the Northern Baltic Sea

Similar to birds, bats also perform long-distance migration between their breeding and wintering sites. In Northern Europe, migratory bat species are often detected along the coastline of the Baltic Sea particularly during migration seasons in the spring and autumn. In spite of regular monitoring of bat migration at coastal sites, the overall distribution of migratory bats in Northern Europe and variability between sites and seasons are still very poorly known. In this study we used automated bat detectors to compare the activity of migratory bat species between coastal and inland monitoring sites along the west coast of Finland (61.5-61.9 degrees N, 21.3-22.3 degrees E). Our main goal was to test whether the activity of migratory bat species is associated with the coastline or whether these species also occur inland. Of migratory bat species observations, 98.6% were covered by Pipistrellus nathusii, which was detected at all our monitoring sites. The activity of the species decreased rapidly, with increasing distance from the coastline towards inland, indicating a sharp activity gradient along the coastline of the Baltic Sea. Because the activity of P. nathusii occurred in migration season and no similar spatial pattern was detected among sedentary species, our results suggest that the aggregation of P. nathusii at the coastline is related to migration as such rather than regular foraging behavior of this species. Our study has direct implications to the wind power planning in Northern Europe. Based on our study we conclude that the impact of wind power on both migratory (namely P. nathusii) and sedentary bat species (namely Eptesicus nilssonii) should be taken into account in wind power planning and impact mitigation in Northern Europe, especially if new wind farms are located along the coastline of the Baltic Sea. When the turbines are located further inland, more attention in the planning process should however be given to the sedentary bat E. nilssonii.