By day, Brett Thelen and Susie Spikol work at the Harris Center for Conservation Education, engaging the community in learning about and protecting wildlife.

By night, they volunteer for Bat Counts, a cooperative project of New Hampshire Fish and Game and the University of New Hampshire Cooperative Extension to monitor summer bat populations.

That’s what has brought them and a colleague, T. Parker Schuerman from New Hampshire Audubon, to this building off a dirt road in the woods of southwestern New Hampshire.

The building houses a maternity colony, where female bats roost in communities to birth, nurse and wean their young, which are called pups. These sorts of old structures are favorite roosting spots for little brown bats and big brown bats, two of New Hampshire’s eight native species.

A sign on the old structure reads “Private Property No Trespassing.”

Shortly after 8 p.m., they start to emerge. The animals drop out of the building’s eaves and crevices, then zoom off like furry guided missiles, as the rapt humans record them with clicking counters.

Bats have had a rough go of it in recent years.

A fungal infection called White Nose Syndrome nearly wiped out entire species, killing an estimated 6 million North American bats in less than two decades.

“It was just completely devastating,” says Haley Andreozzi, state wildlife conservation specialist at UNH Cooperative Extension.

For little brown bats here in particular, the threat was catastrophic, causing a 99% population decline, Andreozzi says.

Wildlife experts here have been keeping a close watch on bats ever since. And there are some signs the animals may be starting to recover.

The annual count compiles data from about 20 sites, a mix of barns, outbuildings and bat houses, in six counties — Cheshire, Grafton, Hillsborough, Merrimack, Rockingham and Sullivan — according to Sandra Houghton, wildlife diversity biologist at Fish and Game’s nongame and endangered wildlife program.

Last year, about 60 volunteers participated. The count revealed more than 300 bats at four distinct locations — and the first-ever count of more than 700 bats at one site.

Fish and Game biologists also do their own counts in the wintertime at caves and abandoned mines where bats are known to hibernate — they’re called hibernacula.

The scale and speed of the loss from White Nose Syndrome was stunning, Andreozzi said.

In 2009, state biologists recorded more than 3,200 bats hibernating in four of the largest mines, the highest winter count ever.

Just two years later, they found only 16 bats in those mines. “One of them was completely empty,” Andreozzi said.

Thelen and Spikol have positioned themselves to watch the front of the building, while Schuerman is at the back.

As dusk deepens, the mosquitoes grow thicker, and so do the bats. By 8:30 p.m., they start dropping out more rapidly, as a gibbous moon rises over the trees.

This Monadnock location offers an ideal habitat for bats. There’s a pond nearby, lots of forests surrounding, and — did we mention? — plenty of bugs.

“This is the time of night I start daydreaming about scratching my skin with steel wool,” Thelen says, never taking her eyes off the bats.

“Or a pumice stone,” suggests Spikol.

At the back of the building, Schuerman is recording nearly as many bats going in as coming out. Some of the bats emerging from the front seem to be circling back in his direction.

“Maybe some of the pups are still nursing, so there’s more of them going back in,” muses Thelen.

When it’s too dark to see, the volunteers compare notes.

Thelen counted 185 bats coming out, and Spikol counted 210, so they split the difference at 198. Out back, Schuerman counted 43 bats out and 33 in. They subtract the ins from the outs so they don’t double count, and come up with a total for the night of 185.

“That’s really low for this site,” says Spikol, disappointed.

It seems low, agrees Thelen.

Maybe the bats in this colony didn’t have a successful breeding year, given the abnormal weather last spring. “Or the pups are still nursing so there are not as many coming out,” Thelen says.

They had much better luck at a large barn they visited earlier this summer, also in the Monadnock region. They stood amazed as hundreds of bats poured out of an old barn at dusk.

“It was like a kettle of bats,” Spikol says, her eyes alight.

Eight species of bats make their homes in New Hampshire. All are listed as “species of greatest conservation need” in the state’s Wildlife Action Plan.

Four species of native bats — little brown bat, Eastern small-footed bat, tri-colored bat and Northern long-eared bat — are now listed as state endangered, and the Northern long-eared bat is also listed federally as an endangered species.

Five species of bats overwinter in caves and mines here, while the other three — like so many Granite Staters — head south when it gets cold.

The bats that head south — the hoary, red and silver-haired bats — were not affected by White Nose Syndrome, Houghton says.

It’s the bats that hibernate here that were devastated by the disease, which causes them to awaken from hibernation and seek food, only to starve to death when no insects are found.

Winter counts in the past two years have found hibernating bats in places that were devastated by the syndrome — including tricolored bats at a mine where they were last documented in 2009, according to Fish and Game.

Meanwhile, some recent studies suggest that surviving bats may have developed some resistance to the disease.

Houghton said she views the recent data as encouraging — but “with caution,” she said. “Everything is going to be slow and uncertain,” she says.

“With such a dramatic crash, we don’t know how healthy the population is,” she says.

Maternity colonies here used to number in the thousands, Houghton says. Smaller populations might not have the genetic diversity needed for a healthy population, she says.

Another challenge for bat recovery comes down to biology: Bat moms typically have just one pup a year.

Small mammals such as rabbits have multiple offspring, Andreozzi said. “That’s their evolutionary strategy to survive,” she says.

“Bats have a different strategy: They invest in one offspring, but they live a relatively long time,” she says.

Little brown bats in the wild live 6 to 10 years. In captive settings, some have been documented to live up to 30 years, Andreozzi says.

Bat birth rates are “what makes it more difficult to rebound from a population stress.”

Enlisting Bat Counts volunteers to monitor bats at barns and outbuildings on their own property provides biologists with data they wouldn’t otherwise have access to, Fish and Game’s Houghton says.

The project also creates what she calls “bat ambassadors.”

“You have people out there in their communities who are able to, one, share their enthusiasm, but also give information about bats to other folks,” she says.

Andreozzi from Cooperative Extension says there are only a few criteria to become a Bat Counts volunteer: “They need to know that they have bats, that’s the first step. And then they have to care that they have bats, and be interested and have that curiosity.”

Otherwise, it’s pretty straightforward, she says. “You can sit in a lawn chair in your yard,” she said. “It is done on buggy evenings, but it’s really simple to sit there and count bats.”

It’s worth all the bug bites, last week’s volunteers agreed.

“It’s just really cool to see them fly,” Thelen from the Harris Center says. “Their way of being in the world is so different from our own way of being.

“It’s really exciting to step into their world for an hour.”

Thelen says the catastrophic impact of White Nose Syndrome on New Hampshire’s bat population was a wakeup call for people who cherish them. “That really, I think, rocketed them into people’s consciousness in a new way,” she says. “We started to realize how important they are, their role in the ecosystem.”

In the tropics, she says, bats are pollinators of many flowering plants. Here in the Northeast, “They’re actually the greatest predator of flying insects on the landscape.”

Bats typically eat half their body weight in insects — every night. “That has ecological impacts, but also economic impacts,” Andreozzi says.

That’s something everyone should care about, she says.

“We don’t know what it would mean to lose bat species on the landscape,” Andreozzi says. “There probably are ramifications we couldn’t begin to understand.”

It’s easy to see parallels between what humans and bats have endured in recent years — a catastrophic health threat and an agonizingly slow return to normal.

Houghton, for one, admires the creatures’ fortitude in the face of disaster.

“First of all, you’re incredible, as a mammal that flies,” she says. “But now, you’ve survived this catastrophic disease and you’re still here, even in small numbers.

“That’s something to appreciate.”

• If you have questions about bats — including how to prevent bats from living in your house, or safely evict them if they do get in — visit wildlifehelp.org.