Bat migration on the Baltic coast - Station ornithologique suisse

Bat species have been shown to migrate up to 2,000 km between their maternity roosts in Northern Europe and their wintering grounds in Southern and Western Europe. These species are threatened, among other factors, by the expansion of onshore and offshore wind energy facilities. Such installations are often located along migration routes, such as the Baltic Sea coast, and thus pose a significant collision risk. Small-scale radar systems could provide novel and important insights into the migratory behavior of bats, a crucial step towards developing appropriate conservation measures. The analyzed radar echoes deliver high-resolution, long-term local data on bat migration. In 2023, the Swiss Ornithological Institute developed an algorithm for classifying bats on base of the radar echoes. This algorithm is now to be validated by comparing radar data collected on the Latvian Baltic Sea coast between 2021 and 2024, with a comparative dataset from a thermal imaging camera recorded in 2022. The migration traffic rate (Bats per km and hour) was calculated for both observation methods. A correlation of r = 0.56 was calculated for the overlapping range in a height of 50–150 m. Additionally, the observed values, like the median of ground speed, flight height and calculated Radar Cross Section of the radar data were compared with literature based on the determined phenological and morphological characteristics. The results are in good agreement with published data; however, due to outliers and the strong conservative filtering of the raw data, a misclassification error rate of 20–25% is to be expected. Additionally, the influence of wind on the flight behavior of the recorded bats during autumn migration was analyzed. The median wind speed during observed bat migration with the radar was calculated at 4.9 m/s, with a preference for winds from the southwest. A decline in flight intensity was observed at wind speeds between 7 and 10 m/s. Furthermore, a significant discrepancy in flight altitude was found for different wind directions: the median flight altitude with tail wind was 492 m, while it was significantly lower with head wind. Cross wind had no significant effect. Ground speed values were significantly higher with tail wind than with head wind, and the variability of ground speed was greater than that of air speed. Air speed was about 1.5 times lower than ground speed under tail wind conditions. Under head wind conditions, a significant increase in air speed was observed. These results add to the growing body of evidence that bats adaptively use wind conditions and pursue an active migration strategy, showing flexibility even when wind conditions are not optimal. These results provide a valuable basis for further study of bat migration using data sets from small-scale radars and also show how the data can be used in conjunction with weather data to study migration behavior in order to develop evidence-based measures to reduce bat mortality from wind energy development.