WSB: How to best use acoustic monitoring to survey flying squirrels

Trapping elusive flying squirrels isn’t easy — some research shows biologists have just a 1% success rate. Nest boxes are another option, but they sometimes have to be deployed for several years before they actually work. Camera traps work, but it’s hard to tell different species of flying squirrels apart.

Then biologists found acoustic monitoring, which can detect the unique ultrasonic calls of different species of flying squirrels, proves much more successful. But when is the best time to use acoustic monitoring? And where?

In a recent study published in The Wildlife Society Bulletin, researchers set out to determine what season this method works best for the federally endangered Carolina northern flying squirrel (Glaucomys sabrinus coloratus) and how long to deploy detectors in habitats of varying quality .

“When they were listed, land managers had been using nest boxes to survey populations, but nest boxes take a lot of effort and you have to have them out for several years,” said Corinne Diggins, a research scientist in the Department of Fish and Wildlife Conservation at Virginia Tech and lead author of the study. “Flying squirrels tend to use nest boxes more in low-quality habitat rather than high-quality habitat, because there are not as many nesting or denning options.”

Diggins and her colleagues wanted to refine acoustic monitoring protocol to monitor the squirrels’ occupancy in higher quality habitats over time. Carolina northern flying squirrels live in high–elevation sky islands of spruce and fir, a habitat type that is a Pleistocene relic and one of the most endangered forested ecosystems in the U.S. The extent of the habitat and its quality has decreased due to factors like historic industrial logging, acid rain and Balsam woolly adelgid infestation. And now, a warming climate is a real threat, since the squirrels’ habitat is already at the highest peaks and ridgelines. They have nowhere higher to go for cooler temperatures. “At this point in time, there’s no evidence that the squirrels can travel between sky islands,” Diggins said. “They’re genetically isolated there.”

Diggins and her colleagues put out the same ultrasonic detectors that researchers use for bats. They placed them in the woods up on trees, pointing them toward the understory, making it more likely to capture squirrel sounds. They did this in both low- and high-quality habitat and recorded sounds across all seasons.

“What we found is, basically you can survey them any time of the year,” she said. “Flying squirrels are very social and produce a lot of vocalizations.” But researchers did find there were times that species differentiation was easier. During late May through June, when juvenile squirrels are dispersing from their nests, the animals commonly produce a trill call, making them easily distinguishable from southern flying squirrels (Glaucomys volans). “Those calls are very distinct,” she said. Even the newly described Humboldt’s flying squirrel (Glaucomys oregonensis) has a unique trill call, she said. “If you’re in an area that has both northern and southern flying squirrels and you want to do a rapid assessment, spring is the best time for that.”

Winter may be a little more difficult, she said, not because the squirrels are staying quiet, but because detectors often don’t work in lower temperatures and are less accessible due to snow and ice.

Her team also found how long you need to survey an area depends on the quality of the habitat. In lower-quality habitat, surveys have to be longer to pick up squirrel sounds. “It’s probably that fewer of them are there or they rarely occur there,” she said.

Diggins hopes long-term acoustic monitoring can help determine the endangered flying squirrel’s occupancy on the landscape. Since acoustics can also provide a more rapid assessment, she said it could be used in the future for pre-project monitoring for construction and timber harvesting projects.

“It’s a really cool technique,” she said. “I think bioacoustics is going to be used for a lot of different animals. A lot of animals produce vocalizations and many species of rodents specifically produce ultrasonic calls, and so acoustics could be a way to survey these different species in the future.”

This article features research that was published in a TWS peer-reviewed journal. Individual online access to all TWS journal articles is a benefit of membership. Join TWS now to read the latest in wildlife research.