Bidirectional movements of Nathusius' pipistrelle bats (Pipistrellus nathusii) during autumn at a major migration corridor

Migration is well documented for many species throughout the animal kingdom. Although migration is also a common behaviour in bats, it is rarely studied due to the cryptic nature of the phenomenon. Recoveries of banded individuals have shown that Nathusius' pipistrelles (Pipis-trellus nathusii) can fly more than 2000 km between their summer and winter ranges in Europe, but further details of how and where they move between the endpoints of their seasonal journeys remain elusive. Here, we used three-dimensional acoustic tracking at a coastal migration corridor to elucidate the flight behaviour of Nathusius' pipistrelles during late summer. Analyzing 432 recorded flight trajectories, we show that the majority of bats followed the expected southerly direction, parallel to the coastline, on all nights, and flying at the optimal speed for long-distance travel with minimal energy expenditure. However, on one day with stronger winds, about 20 % of the bats flew in the opposite, i.e. northerly, direction. The observation of a proportion of in-dividuals flying antiparallel to the mass of migrating conspecifics within the same movement corridor highlights that individuals may follow contrasting movement strategies at the same time and place, presumably depending on environmental conditions. We argue that it is possible for Nathusius' pipistrelles to fly back and forth (south and north) during autumn migration, spending more time on this migration corridor than required for a straight one-way flight. This highlights the urgent need to protect migration corridors along coastlines, particularly as wind energy development continues.

Automated summary

Migration is a common behavior in animals, including bats. However, the migration behavior of bats is not well studied due to its cryptic nature. Using acoustic tracking, researchers have discovered that Nathusius' pipistrelles exhibit a specific flight behavior during late summer migration. Most bats fly in the expected southerly direction along the coastline, optimizing their speed for long-distance travel with minimal energy expenditure. However, on a day with stronger winds, about 20% of the bats fly in the opposite, northerly direction. This observation suggests that individual bats may employ different movement strategies depending on environmental conditions.