It’s wintertime and Townsend’s big-eared bats are well out of sight, their long, pointed ears rolled down tightly, and their heart, metabolic and respiratory rates slowed.

Despite the difficulty of finding bats during the colder months, this week local biologists set out into the field to find Townsend’s and the other 12 species in Marin. It’s the fourth year of an interagency monitoring program intended to establish baseline data on seasonal bat roosting and nesting sites, behaviors and species prevalence throughout the county.

“Bats are often underappreciated and understudied,” Dr. Katie Smith, a biologist with the Golden Gate National Parks Conservancy who helps conduct the research, wrote to the Light. “We consider bats to be an indicator species because they can be affected by climate change, habitat loss, roost disturbance, pesticides and insect declines, as well as disease.”

The 2016 State of the Mountain Health Report, a natural resources analysis of the Mount Tamalpais area, pointed out the dearth of data on bats locally. The monitoring program is now one of several collaborative data-collection projects underway in Marin on lands managed by the partners of the One Tam campaign, including the National Park Service, Marin County Parks, California State Parks, Marin Municipal Water District and the conservancy.

The results will be published in the next iteration of the mountain health report, due out in 2022.

There are several aspects of the monitoring effort. Summertime acoustic monitoring at 37 sites has found at least 13 out of the state’s 25 bat species. That data is sent to the North American Bat Monitoring Program, operated by the United States Geological Survey.

Marin biologists have also conducted several winter and summer radio telemetry studies—whereby tiny transmitters are glued to the backs of bats caught in mist nets—in order to determine which types of habitats bats are using at different times of the year.

Summertime roosts identify the areas essential for breeding. In the Point Reyes National Seashore, two known maternity roosts are the Randall House, an abandoned farmhouse off of Highway 1 in the Olema Valley, and an uninhabited structure at Commonweal in Bolinas.

(The so-called Randall House was historically home to generations of dairy farmers, beginning with Vermonter Sarah Seaver Randall, whose husband William Edgar Randall purchased the land in the mid 1800s and was subsequently murdered by a neighbor; although the two-story Victorian was first deemed eligible for the National Register of Historic Places in 1979 for its rich cultural legacy, the park service was ready to demolish the house—which hasn’t seen a human occupant for decades—before a colony of Townsend's was discovered there in the ‘80s.)

With over 1,200 known species of bats worldwide, breeding behaviors vary. For the Townsend's, groups of around 100 females cluster together between the months of March and June in dark, open spaces while the males roam; their pups are usually born by July and, three weeks later, start to fly. Like most species, the females have one pup per year, though the overall lifespan—an average of 16 years—is relatively short, considering other species of bats can live up to 40 years.

Not all bat species hibernate in winter, and some of the species found in Marin migrate elsewhere to do so. Those that do retreat in the colder months are generally known to prefer spots that remain dark and at a constant temperature despite fluctuations outside, such as caves, canyons, crevices between rocks—and, increasingly, within buildings, storm sewers and mines. These sites are often different from those preferred for breeding and also depend on the different behaviors of the individual species.

The 12 other species that have been identified in the county—many of which are often found in same locations—include the big brown bat, the little brown myotis, the fringed myotis, the California myotis, the long-legged myotis, the Yuma myotis, the Western red bat, the hoary bat, the pallid bat, the Western long-eared bat, the silver-haired bat and the Mexican free-tailed bat.

Details on the wintertime hideouts are significant for the county’s preemptive evaluation of local populations’ susceptibility to white-nose syndrome, the deadly disease that has wiped out 6 million bats nationwide since it was detected in 2006.

Health assessments have not found any bats in Marin infected by the fungus that causes the disease, Pseudogymnoascus destructans. (The first case on the West Coast was found in Washington State in 2016, and last year, three bats sampled in Chester, Calif., had positive, though low-level, results for the fungal DNA.)

“The big unknown about white-nose syndrome and how it will affect bats in the West depends on what they are doing in the winter,” said Dr. Brian Halstead, a research wildlife biologist with the U.S.G.S., which has a full-time field biologist helping Marin and other California counties conduct surveys.

“We want to know what the environmental characteristics of these wintertime roosts in California are. What is the humidity and temperature like? And are those conditions [beneficial] to [the fungus’s] growth?” Dr. Halstead asked.

Currently, he said field biologists are doing their best to find the bats’ winter roosts. In the summer, bats spend their nights foraging and are more easily caught for tracking; now, even if biologists do catch a bat out and about, the radio transmitters used to track where they are hibernating remain problematic, Dr. Halstead said.

“They are so light that we can only put really small transmitters on them, so the battery doesn’t last very long or transmit very far. That’s fine if, say, you’re tracking a frog that doesn’t go very far, but with a bat moving in the night it becomes really tricky to find them,” he said.

Once the monitoring program collects enough data to determine a baseline understanding of local bat populations, Dr. Smith said “we hope to be able to detect and monitor any changes over time. We hope that our data will help guide land managers to better conserve bats, and the habitats they rely on.”