Tracking tiny flight paths
Bat migration mystery may be solved with miniature transmitters
Scientists in Wisconsin have successfully tracked the state’s smallest bat using an ultra-light transmitter that beamed its movement as the bat emerged from hibernation.
The findings are significant and come at a time when researchers look for ways to best protect the eastern pipistrelle as its numbers plummet.
The Department of Natural Resources said biologists and volunteers followed a female eastern pipistrelle — which weighs less than a nickel — from a cave in Pierce County in early May to a roosting spot along the Mississippi River.
The flight path: A virtual beeline to the river, where it flitted between the shorelines of Wisconsin and Minnesota and then settled into a birch tree to roost. In all, the bat flew about 35 miles in a little more than three hours.
It was only one bat, but the results are expected to help the DNR better understand the behavior of
the eastern pipistrelle, which is listed as a state threatened species and is highly vulnerable to white-nose syndrome, a deadly bat disease sweeping the United States.
The disease is killing millions of insect-eating bats. If scientists know more about their habit and migratory path, agencies such as the DNR have a better chance of fine-tuning their protective measures during critical periods when adults are rearing their young, known as “pups.”
Knowing more about the migration of the eastern pipistrelle will help the agency with its conservation work by protecting the bats in known locations and helping avoid limits on land use such as logging over broad areas.
The May experiment was the first time anyone had ever successfully tracked an eastern pipistrelle with the use of telemetry during spring migration.
Last year, the DNR tried the same experiment. But the bats, also known as tri-colored bats, hibernated longer than expected and busy biologists were forced to get back to their other duties.
Scientists have used radio telemetry to track wildlife for decades.
But when the subject weighs only 6 to 8 grams, there are challenges in having a transmitter and battery light durable enough to transmit its whereabouts, said Paul White, a mammal ecologist who manages the DNR’s bat program.
“There has been a technological bottleneck,” White said.
With a fine mesh net placed over the entrance of a cave, biologists and volunteers this year successfully netted two females. The bats were weighed and outfitted with transmitters. They were released about 11 p.m on May 5. And then it was off to the races. Both bats flew straight west. But the six teams — driver and tracker in each vehicle — quickly lost one of the bats.
By 12:15 a.m. both signals were lost.
Then an hour later, DNR conservation biologist Katie Luukkonen picked up the signal of one of the bats.
Other teams converged and spent the next two hours tracking it as it flew to the Mississippi, then upriver until it settled in a tree north of Hudson.
Luukkonen and fellow conservation biologist Heather Kaarakka kept an eye on it for the rest of the night.
They returned for the next few nights to see whether the bat was staying put.
The telemetry results suggest the eastern pipistrelle moves the most in the first few days after hibernation, White said. If this bat is representative of the species, they don’t appear to travel far.
By contrast, another bat threatened by white-nose syndrome, the little brown, is known to fly up to 290 miles after hibernation, he said.
Since the eastern pipistrelle is a state threatened species, the DNR must protect known roost areas during key periods in the summer when pups can’t fly and are most vulnerable.
“If we can understand land use and spring migratory behavior, it will help us better manage areas that are important for the survival of this species,” White said.
The work is taking place as bats are being decimated by white-nose syndrome.
The disease produces a powdery white fungus on the nose, ears and wings. The fungus causes the bats to wake up more often during hibernation, burning up more energy, eventually leading to dehydration and starvation.
In 2011, as white-nose syndrome was beginning to spread, Wisconsin officials added the eastern pipistrelle, the little brown, northern long-eared and big brown bats to a state list of threatened species.
White said eastern pipistrelles in a cave in Grant County in the fourth year of infection suffered a 94% population loss this year.
At eight other sites, where the bats are in their third year of infection, numbers have dropped 72%.