Much is known about bird movements across Europe, but the migration flights of common noctule bats were passing under the scientists’ radar until they developed innovative new sensory tags.
The hazardous winter and summer journeys of mainly females of the species are now beginning to be understood thanks to these tiny tags. Scientists from the Max Planck Institute of Animal Behavior(MPI-AB) on the Swiss/German border have attached them to the bats in Switzerland every spring for the past three years. They sit between the bats’ shoulder blades and transmit long-range data, taking1,440 daily sensor measurements of the animals’ activities and their surrounding temperatures.
This has built up a compelling picture of how they travel to their summer roosts in northern Europe,to mate and raise their young. The females tend to be the ones to relocate each year; the males largely stay in one place. In winter they hibernate in tree hollows, singly or in bachelor groups.
“The tags communicate with us from wherever the bats are because they have coverage across Europe much like a cell phone network,” says Timm Wild, who led the development of the ICARUS-TinyFoxBatt tag.
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It transpires that, unlike birds, bats do not have a defined route or migration corridor to follow. “We had assumed that bats were following a unified path,” says Dina Dechmann from MPI-AB, “but we now see they are moving all over the landscape in a general northeast direction.”
The next thing the researchers discovered was that the bats could fly up to 400km in just one night, which set a new record greater than previously observed. Between those long stretches of flight the animals were taking frequent breaks.
“Unlike migratory birds, bats don’t gain weight in preparation for migration,” says Dechmann. “They need to refuel every night, so their migration has a hopping pattern rather than a straight shot.”
Then came the truly astonishing discovery. As the scientists analysed the data they noticed a striking pattern was developing. “On certain nights, we saw an explosion of departures that looked like bat fireworks,” says Edward Hurme, a postdoctoral researcher at MPI-AB who took the lead on the study. “We needed to figure out what all these bats were responding to on those particular nights.”
What they found was that the ‘explosions’ of bats coincided with changes for the worse in the weather. On nights when air pressures dropped and temperatures spiked the bats would make a mass exit from their roosts to continue their migration. They were leaving just before a storm arrived.
“They were riding storm fronts, using the support of warm tailwinds,” says Hurme. It was obvious from the tags’ sensors that the bats were using less energy by flying ahead of the weather front, surfing on the waves of warm air being pushed ahead of the storm.
Aside from greatly adding to the knowledge of Common Noctule behaviour, this discovery could also result in an important conservation measure. Although these bats are not currently an endangered species, all bats are suffering from loss of roosting and feeding habitat and facing modern hazards such as collisions with wind turbines.
One small but significant result of the study could be a change in practice at wind farms. Knowing that squadrons of migrating bats will likely be passing through an area just before a storm, the farms could turn off their turbines for a period to allow them safe passage.
