Migrating bats rely on storm winds to conserve energy

By Stephen Beech

Bats are "riders on the storm" when they migrate thousands of miles across continents, reveals new research.

The winged mammals surf storm fronts as they traverse Europe, North America and Africa, according to the findings.

Scientists say the behavior is rare and difficult to observe - which is why long-distance bat migration has remained an enigma until now.

Researchers from the Max Planck Institute of Animal Behavior (MPI-AB) in Germany have studied 71 common noctule bats on their spring migration across Europe.

Ultra-lightweight, intelligent sensors attached to bats uncovered a method used by the tiny mammals for travel.

The findings, published in the journal Science, show that bats surf the warm fronts of storms to fly further with less energy.

Study first author Dr. Edward Hurme said: “The sensor data are amazing.

“We don’t just see the path that bats took, we also see what they experienced in the environment as they migrated.

"It’s this context that gives us insight into the crucial decisions that bats made during their costly and dangerous journeys.”

Using state-of-the-art sensor technology, the study examined a portion of the total migration of noctules, which scientists estimate to be around 1,000 miles.

Dr. Hurme said: “We are still far from observing the complete yearly cycle of long-distance bat migration.

"The behavior is still a black box, but at least we have a tool that has shed some light.”

The study’s tracking device was developed by engineers at MPI-AB.

Weighing just 5% of the bat’s total body mass, the tiny tag includes multiple sensors that recorded activity levels of bats and temperature of the surrounding air.

Normally, scientists would need to find tagged animals and be close enough to download such detailed data.

But the study’s tag compressed the data, totaling 1,440 daily sensor measurements, into a 12-byte message that was transmitted via a novel long-range network.

Senior author Dr. Timm Wild, who led the development of the ICARUS-TinyFoxBatt tag, said: “The tags communicate with us from wherever the bats are because they have coverage across Europe much like a cell phone network.”

The team deployed the tags on common noctules, a bat that is wide-spread in Europe and one of only four bat species known to migrate across the continent.

Every spring for three years, the scientists attached tags on common noctules in Switzerland, focusing exclusively on females which are more migratory than males.

Females spend summers in northern Europe and winters in a range of southerly locations where they hibernate until Spring.

The tags collected data for up to four weeks as the female noctules migrated back north-east, revealing trajectories far more variable than previously thought.

Senior author Dr. Dina Dechmann, of MPI-AB, said: “There is no migration corridor.

“We had assumed that bats were following a unified path, but we now see they are moving all over the landscape in a general north-east direction.”

The research team were able to distinguish hour-long feeding flights from the much longer migratory flights, finding that noctules can migrate almost 250 miles in a single night - breaking the known record for the species.

Bats alternated their migratory flights with frequent stops, likely because they needed to feed continuously.

Dr. Dechmann said: “Unlike migratory birds, bats don’t gain weight in preparation for migration.

“They need to refuel every night, so their migration has a hopping pattern rather than a straight shot.”

The research team were able to detect a "striking" pattern.

Dr. Hurme said: “On certain nights, we saw an explosion of departures that looked like bat fireworks.

“We needed to figure out what all these bats were responding to on those particular nights.”

They found that the migration waves could be explained by changes in weather.

Bats left on nights when air pressure dropped and temperature spiked; in other words, the bats left before incoming storms.

Dr. Hurme said: “They were riding storm fronts, using the support of warm tailwinds."

The tag’s sensors that measured activity levels also showed that bats used less energy flying on nights of warm wind, confirming that the tiny mammals were harvesting invisible energy from the environment to power their continental flights.

Dr. Hurme said: “It was known that birds use wind support during migration, and now we see that bats do too."

The researchers say their findings go beyond biological insight.

They warned that migratory bats are threatened by human activity, in particular wind turbines which are the cause of frequent collisions.

The researchers say that knowing where bats will be migrating, and when, could help to prevent deaths.

Dr Hurme said: “Before this study, we didn’t know what triggered bats to start migrating.

“More studies like this will pave the way for a system to forecast bat migration.

"We can be stewards of bats, helping wind farms to turn off their turbines on nights when bats are streaming through."

He added: "This is just a small glimpse of what we will find if we all keep working to open that black box.”