Behavior of Scandinavian Bats during Migration and Foraging at Sea

TLDR

Bats likely rely on echoes from the water surface to navigate during long‑distance migration. The study used direct observation and acoustic recordings to track sea‑migrating bats. Eleven species were observed flying up to 14 km offshore, using low‑frequency sonar at low altitudes (<10 m) and foraging on insects and crustaceans, with altitude adjustments near structures that highlight a risk of mortality from offshore wind turbines.

Abstract

We studied bats migrating and foraging over the sea by direct observations and automatic acoustic recording. We recorded 11 species (of a community of 18 species) flying over the ocean up to 14 km from the shore. All bats used sonar during migration flights at sea, often with slightly lower frequencies and longer pulse intervals compared to those used over land. The altitude used for migration flight was most often <10 m above sea level. Bats must use other sensory systems for long-distance navigation, but they probably use echoes from the water surface to orient to the immediate surroundings. Both migrant and resident bats foraged over the sea in areas with an abundance of insects in the air and crustaceans in the surface waters. When hunting insects near vertical objects such as lighthouses and wind turbines, bats rapidly changed altitude, for example, to forage around turbine blades. The findings illustrate why and how bats might be exposed to additional mortality by offshore wind power.