Birds are widely recognized as the ultimate travelers in nature, but they’re not the only ones capable of making extensive journeys. Certain species of bats undertake impressive migrations, covering thousands of kilometers across North America, Europe, and Africa. This behavior is infrequent and challenging to observe, which has contributed to the mystery surrounding long-distance bat migration. Recently, researchers have studied 71 common noctule bats during their spring migration throughout Europe, significantly advancing our understanding of this elusive behavior. By using ultra-lightweight, advanced sensors attached to these bats, they discovered that these small creatures employ a unique strategy during their travels: they ride the warm fronts of storms to enhance their distance while conserving energy. This research has been published in Science.
Birds are widely recognized as the ultimate travelers in nature, but they’re not the only ones capable of making extensive journeys. Certain species of bats undertake impressive migrations, covering thousands of kilometers across North America, Europe, and Africa. This behavior is infrequent and challenging to observe, which has contributed to the mystery surrounding long-distance bat migration. Recently, researchers from the Max Planck Institute of Animal Behavior (MPI-AB) have studied 71 common noctule bats during their spring migration throughout Europe, significantly advancing our understanding of this elusive behavior. By using ultra-lightweight, advanced sensors attached to these bats, they discovered that these small creatures employ a unique strategy during their travels: they ride the warm fronts of storms to enhance their distance while conserving energy. This research has been published in Science.
“The data collected from the sensors are incredible!” exclaims lead author Edward Hurme, a postdoctoral researcher at MPI-AB and the Cluster of Excellence Collective Behaviour at the University of Konstanz. “We not only tracked the paths taken by the bats but also gathered insights into their environmental experiences throughout their migration. This context helps us understand the critical decisions bats made during their demanding and risky journeys.”
Employing innovative sensor technology, the study focused on a segment of the noctule’s entire migration, which scientists estimate to be around 1600 kilometers. “We are still a long way from capturing the whole yearly cycle of long-distance bat migration,” Hurme notes. “The various behaviors are still somewhat unclear, but this new tool has illuminated some aspects.”
The tracking device was created by engineers at MPI-AB. This tiny tag, which constitutes only five percent of the bat’s body weight, contains multiple sensors to monitor the bats’ activity levels and the temperature of the air around them. Traditionally, researchers would need to recapture tagged animals to extract this detailed information. However, the tags condensed data, totaling 1440 daily sensor readings, into a 12-byte message that could be sent through a novel long-range network. “The tags communicate with us from anywhere the bats fly, thanks to coverage across Europe similar to that of a cell phone network,” explains senior author Timm Wild, who spearheaded the development of the ICARUS-TinyFoxBatt tag at MPI-AB.
The research team equipped common noctules—widespread in Europe and one of just four bat species known to migrate across the continent—with these sensors. For three consecutive years each spring, the scientists attached tags to female noctules, focusing on this gender due to their greater migratory tendencies. These females spend their summers in northern Europe and migrate to various southern regions to hibernate during the winter, returning in the spring.
The tags recorded data for up to four weeks as the female noctules migrated back to the northeast, revealing that their travel paths were much more varied than previously assumed. “There isn’t a specific migration corridor,” senior author Dina Dechmann from MPI-AB states. “We had thought bats were following a single route, but we now see they spread out across the landscape while generally heading northeast.”
By analyzing the data, the researchers could differentiate between hour-long feeding flights and the much longer migratory flights, finding that noctules could cover nearly 400 kilometers in just one night, setting a new record for the species. The bats frequently alternated their long migrations with brief stops, likely due to their need to continuously feed. “Unlike migratory birds, bats don’t accumulate extra weight before migration,” Dechmann explains. “Instead, they have a pattern of hopping rather than a straight route because they need to refuel each night.”
Unexpectedly, the researchers discovered a notable pattern. “On certain nights, we witnessed a surge of departures like bat fireworks,” Hurme reports. “We had to determine the factors causing such a response among the bats on those nights.”
They found that these migration bursts could be attributed to weather changes. Bats tended to leave on nights when air pressure dropped and temperatures increased, meaning they took off ahead of approaching storms. “They were taking advantage of storm fronts, utilizing the lift from warm tailwinds,” Hurme explains. The activity level sensors also indicated that bats expended less energy flying on these nights with warm winds, confirming that the bats tapped into surrounding energy to facilitate their long journeys. “It was previously recognized that birds benefit from wind assistance while migrating, and now we’ve confirmed that bats do as well,” he adds.
The implications of these findings stretch beyond merely understanding this little-explored behavior. Migratory bats face threats from human activities, especially from wind turbines that cause frequent collisions. By predicting when and where bats will migrate, it’s possible to reduce fatalities.
“Prior to this research, we were uncertain about what instigated bats to start their migrations,” remarks Hurme. “Continued studies in this area will help develop a system for forecasting bat migration. We can protect bats by advising wind farms to shut down their turbines on nights when large numbers are migrating. This is just the beginning of our exploration into this previously mysterious topic.”
