“Surfing the Storm: Bats Ride the Winds on Their Grand Migration”

“The data from the sensors are astonishing!” expresses lead author Edward Hurme, a postdoctoral researcher at MPI-AB and part of the Cluster of Excellence Collective Behaviour at the University of Konstanz. “We not only track the route taken by the bats but also capture what they encounter in their environment while migrating. This context provides us with insights into the vital choices bats made during their demanding and perilous journeys.”

Utilizing innovative sensor technology, the research focused on a segment of the total migration undertaken by noctules, which scientists estimate to be approximately 1600 kilometers. “We are still far from observing the entire annual cycle of long-distance bat migration,” notes Hurme. “The behavior remains largely unknown, but we now possess a tool that has illuminated some aspects.”

The tracking device utilized in the study was engineered by specialists at MPI-AB. Weighing merely five percent of the bat’s overall body weight, the compact device features numerous sensors that documented the activity levels of the bats and the surrounding air temperature. Typically, researchers would need to locate tagged animals and be sufficiently nearby to download such detailed information. However, the tagging device in this study compressed the data, amounting to 1440 daily sensor readings, into a 12-byte message transmitted through an innovative long-range network. “The tags communicate with us from wherever the bats find themselves since they have coverage across Europe similar to a mobile network,” explains senior author Timm Wild, who spearheaded the development of the ICARUS-TinyFoxBatt tag in his Animal-borne Sensor Networks group at MPI-AB.

The research team deployed the tags on common noctules, a bat species that is prevalent in Europe and one of only four species recognized to migrate across the continent. Every spring for three years, the scientists affixed tags to common noctules in Switzerland, concentrating solely on females, which are more migratory than their male counterparts. Females spend their summers in northern Europe and their winters in various southern locales where they hibernate until spring.

The tags gathered data for up to four weeks as the female noctules migrated north-eastward, revealing flight paths that were considerably more complex than previously understood. “There is no singular migration corridor,” asserts senior author Dina Dechmann from MPI-AB. “We had presumed that the bats followed a consistent route, but we now observe that they navigate across the landscape in a general northeastern direction.”

The scientists separated the data to differentiate between hour-long feeding flights and the significantly longer migratory flights, discovering that noctules can cover nearly 400 kilometers in a single night — surpassing the existing record for the species. Bats alternated their migratory flights with frequent stops, likely due to the need for continuous feeding. “In contrast to migratory birds, bats do not gain weight in preparation for their migration,” comments Dechmann. “They must refuel nightly, resulting in a migration pattern that resembles hopping rather than a straight line.”

The researchers then identified a remarkable trend. “On specific nights, we observed a surge of departures that resembled a fireworks display of bats,” shares Hurme. “We needed to determine what was triggering all these bats to act on those particular nights.”

They discovered that these migratory surges could be attributed to changes in weather conditions. Bats departed on evenings when air pressure fell and temperatures rose; in essence, the bats left ahead of oncoming storms. “They were utilizing storm fronts, taking advantage of warm tailwinds,” states Hurme. The tag’s sensors measuring activity levels further indicated that bats expended less energy flying during these warm wind nights, confirming that these minuscule mammals were capitalizing on unseen energy from their surroundings to fuel their continental migrations. “It was already known that birds harness wind assistance during migration, and now we observe that bats do likewise,” he adds.

The ramifications of these discoveries extend beyond biological understanding of this overlooked behavior. Migratory bats face threats from human activities, particularly from wind turbines that cause frequent fatalities. Understanding where and when bats will migrate could aid in preventing their deaths.

“Prior to this research, we had no knowledge of what initiated bats to commence their migration,” states Hurme. “Further investigations like this will enable us to develop a system to predict bat migration. We can be caretakers of bats, assisting wind farms in shutting down their turbines on evenings when bats are moving through. This is merely a brief glimpse of what we will uncover if we persist in our efforts to unveil that enigma.”