“Surfing the Skies: Bats Riding Storm Fronts on Their Epic Migration”

Birds may be the undisputed kings of long-distance flight—but they are not the sole long-haul travelers. A select few bats are known to traverse thousands of kilometers during continental migrations across North America, Europe, and Africa. This behavior is infrequent and challenging to observe, which is why long-distance bat migrations remain a puzzle. Recently, researchers from the Max Planck Institute of Animal Behavior (MPI-AB) studied 71 common noctule bats during their spring migration across Europe, offering significant advancements in understanding this enigmatic behavior. Ultra-light, smart sensors affixed to the bats uncovered a method employed by these small mammals for travel: they ride the warm fronts of storms, allowing them to fly further with reduced energy expenditure. The findings are published in Science.

“The sensor data is extraordinary!” remarks 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 don’t simply trace the route the bats took; we also learn what they encountered in their environment during migration. This context helps us understand the critical decisions the bats made throughout their taxing and perilous journeys.”

Employing innovative sensor technology, the study investigated a segment of the total journey of noctules, estimated by scientists to be around 1600 kilometers. “We are still distant from capturing the entire yearly cycle of long-distance bat migration,” states Hurme. “The behavior remains somewhat of a mystery, but at least we possess a tool that has illuminated some aspects.”

The tracking device utilized in the study was engineered by specialists at MPI-AB. Weighing only five percent of the bat’s overall body weight, the miniature tag encompasses various sensors that monitored the bats’ activity levels and the ambient air temperature. Typically, scientists need to locate tagged animals and be sufficiently close to retrieve such detailed information. However, this study’s tag condensed the data, comprising 1440 daily sensor records, into a 12-byte message transmitted via a cutting-edge long-range network. “The tags communicate with us from wherever the bats might be since they have coverage throughout Europe, similar to a cell phone network,” explains senior author Timm Wild, who spearheaded the development of the ICARUS-TinyFoxBatt tag within his Animal-borne Sensor Networks team at MPI-AB.

The researchers deployed the tags on common noctules, a bat species prevalent across Europe and among the few known to migrate throughout the continent. Each spring for three years, the scientists attached tags to common noctules in Switzerland, concentrating solely on females, which display more migratory behavior than males. Females spend their summers in northern Europe and winters in various southern locations where they hibernate until spring.

The tags collected data for up to four weeks as the female noctules migrated back toward the northeast, showcasing trajectories that were much more variable than previously assumed. “There is no fixed migration corridor,” states senior author Dina Dechmann from MPI-AB. “We had presumed that the bats were following a singular pathway, but we now observe they are navigating across the landscape generally toward the northeast.”

The scientists analyzed the data to differentiate hour-long feeding flights from the significantly longer migratory journeys, discovering that noctules can travel nearly 400 kilometers in a single night—setting a new record for the species. The bats alternated their migratory flights with frequent pauses, likely owing to their need to feed continuously. “In contrast to migratory birds, bats do not gain weight in preparation for migration,” notes Dechmann. “They require nightly refueling, resulting in a hopping migration pattern rather than a direct route.”

The authors then uncovered a remarkable trend. “On specific nights, we observed a surge of departures that resembled bat fireworks,” mentions Hurme. “We needed to determine what triggered all these bats on those particular evenings.”

They discovered that these waves of migration could be linked to variations in weather conditions. Bats departed on nights when air pressure declined and temperatures increased; in essence, they took off before incoming storms. “They were gliding on storm fronts, utilizing the assistance of warm tailwinds,” Hurme elaborates. The sensors measuring activity levels further indicated that bats expended less energy flying on those warm wind nights, confirming that these small mammals were harnessing unseen energy from their environment to fuel their transcontinental journeys. “It was already established that birds leverage wind support during migration, and now we realize bats do as well,” 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 which often result in collisions. Knowing where bats will be migrating, and the timing, could aid in preventing fatalities.

“Prior to this study, we were unaware of what instigated bats to commence migrating,” states Hurme. “Further research of this nature will pave the way for a system to anticipate bat migration. We can act as guardians of bats, providing assistance to wind farms in ceasing turbine operations on nights when bats are moving through. This is merely a small preview of what we will uncover as we persist in our efforts to unveil that mystery.”

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