Daytime songbird migrants at sea: the influence of coastal proximity on abundance

Georg Rüppel, Clara Liehmann, Kai Borkenhagen, Stefan Garthe, Nele Markones, Heiko Schmaljohann, Thiemo Karwinkel

Human activities at sea, including offshore energy development, along with environmental changes, are altering marine ecosystems. Their impact on land-based migratory species crossing marine areas remains poorly understood. Songbirds frequently traverse even remote ocean areas, facing risks such as drowning at sea and interactions with man-made structures. Yet, their spatial distribution at sea, critical for assessing potential threats, remains largely unquantified. Using a 30-year dataset of ship-based observations, we map the large-scale marine distribution of mainly daytime songbird migrants in German waters. Despite their regular offshore occurrence, even in large flocks, migration intensity declined with increasing distance from the coast and consistently across regions (North and Baltic Sea), seasons (spring and autumn), and the abundant species. Due to observational challenges, nighttime migrants are underrepresented, but we assume a similar distribution pattern. When integrated with radar surveys, individual tracking, and phenological data, these insights inform conservation strategies as offshore developments expand.