Quantifying Migratory Bat Movements in Central Europe Across Seasons and Years Using a Vertical-Looking Radar

Bat migration is an ecologically important yet poorly understood phenomenon. This is in part because monitoring these migrations is challenging, due to bats' nocturnal behaviors and their sometimes high-altitude migratory flights. This study presents the first radar-based examination of multi-annual migratory bat phenology in Europe, utilizing vertical-looking radar data collected on the Swabian Plateau in Germany between September 2019 and December 2022. Bat activity was consistently low in winter and increased gradually from March onwards to a peak between July and September. Across all years, pre-maternity migration began between late February and mid-March, while post-maternity migration ended between late October and mid-November. We estimated peak radar-based migration traffic rates between 1159 and 2473 bats per km, with the highest peak recorded on 4 July 2022. Correlations between radar-derived nightly bat numbers and simultaneously acquired acoustic recordings ranged from 0.47 to 0.70 for the pre-maternity season, and from 0.14 to 0.71 during post-maternity migration. Both monitoring techniques showed peak bat activity during the summer, with smaller surges in September and October. The radar, however, detected significantly more bats overall. These findings showcase how vertical-looking radars can be used to quantify and characterize seasonal variability in high-altitude bat movements. Through strategic future radar deployments and the analysis of available historical datasets, our current understanding of migratory bat seasonality, routes, and intensity could increase drastically, and underpin the development of effective protocols for biodiversity conservation.