Researcher keeps tabs on bats

Counting a few hundred thousand bats sounds about as easy as herding a few thousand cats, but a university researcher is successfully tallying the winter residents at one of the largest bat hibernation sites in the Midwest.

Hundreds of thousands of little brown bats, along with northern long-eared bats, eastern pipistrelles, and big brown bats, use the Neda Mine as their winter home. The old iron mine, abandoned in 1914, sits at the southern end of the Niagara Escarpment, a bedrock ridge running from Dodge County north through Door County. The escarpment, standing above glaciated plains and lowlands, is an important habitat and migration corridor for wildlife. It’s also under considerable development pressure.

“There is concern over the status of bats in Wisconsin, but the data are difficult to get because it’s difficult to census bats,” says Scott Craven, chair of the Wildlife Ecology Department at the College of Agricultural and Life Sciences. “Development and pesticides may be threats, but it hasn’t been easy to document.”

Research at the Neda Mine is filling in some of the blanks in our knowledge of bats’ behavioral ecology.

“For conservation planners to make informed decisions, the first step in assessing threats to bat populations is to have accurate censuses of their abundance over time,” says Dave Redell, a wildlife ecology research assistant at the College of Agricultural and Life Sciences.

It’s difficult to accurately count bats inside the Neda Mine – the animals roost in cracks and fissures inaccessible to people, and unstable rock makes the mine’s four miles of tunnels hazardous to human health. Counting swarms of free-flying bats in the open air isn’t practical.

Redell decided to take head counts at the mine entrances. Most entryways are sealed, but four are fitted with gates that admit bats but exclude humans. These gates also sport electronic bat counters that monitor the entrances 24 hours a day, counting each bat that exits or enters.

The counters use photo beam-break detectors, shining beams of invisible infrared light across the bats’ flight path. Each light beam shines onto two photocells, allowing the system to determine direction of flight by which cell goes dark first. The system at each gate can detect up to 1,000 passages per second.

Redell has calibrated the detectors by comparing their results with images from an infrared video camera. “A painful frame-by-frame comparison of video to beam-break gives us an error rate, so we can say that there are X number of bats, plus or minus X. This gives us a baseline for long-term monitoring, so we can say whether the population is increasing, decreasing or holding steady,” he says.

The detectors, along with environmental monitors, are connected to an on-line data logger. From his desk on campus, Redell can download up-to-the-minute information on activity and conditions at the mine. He’s processing detector data now, and plans to have a final estimate of the mine’s bat population by the end of summer.

Previous estimates have placed the mine’s winter bat population at 300,000 to 500,000. Redell says preliminary measurements from 2001 indicated about 150,000 to 200,000 bats used the mine, but believes that combining data from 2001 and 2002 will give a more reliable estimate.

Bats enter the mine from August through November, and leave in April and May. By the time spring arrives, hibernating bats have used up most of their fat reserves. They can’t afford to make a lot of trips outside to see if conditions are right for spring dispersal. Conditions remain fairly constant within mines and caves, so Redell thinks the bats use some cue that allows them to indirectly monitor outside conditions without having to fly outside to check.

“A powerful tool for managers involved with bat conservation to have is the ability to predict whether there will be a large number of bats leaving a hibernaculum on any particular night,” Redell says. “We want to find out what environmental factors bats are cueing in on. In other words, can we say that, given certain environmental conditions, bats are likely to leave the mine tonight?

“We hope to determine which environmental conditions have predictive power. Knowing this, we can remotely download environmental data, analyze it, and be able to say ‘The bats are likely, or not likely, to move out tonight.'” Redell’s data from 2000 and 2001 indicate that the bats may be cueing in on airflow direction within the mine; he intends to further test this hypothesis in 2002.

Areas near the escarpment have been studied as potential “wind farm” sites for commercial energy generation. Bats usually lose when they tangle with wind turbines. “If necessary, by having the ability to predict peak emergences, it may be possible to shut off the wind generators during these peak nights,” Redell says.

Redell has used an array of ultrasound detectors set up on the lands around the mine to record the bats’ echolocation calls. Data from the detectors allowed Redell to chart where the bats headed when they left the mine, and which landscape features — such as open areas, wood lots, or tree-line edges — they related to as they flew off. During spring migration, the bats consistently used the Niagara Escarpment as a travel corridor – information that could be crucial in siting wind generators.

Redell’s project is part of an ongoing research partnership involving the UW–Madison and UW-Milwaukee, the Wisconsin Department of Natural Resources, and the Max McGraw Wildlife Foundation.