Bay Nature magazineJuly-September 2001


Shadows in Flight

July 1, 2001

Ten hours ago, my hillside perch high above Highway 1 offered a sweeping, sunlit view across Inverness Ridge and seaward into the heart of Point Reyes National Seashore. Five hours ago, as night closed in, familiar landmarks like Firtop and Mount Wittenberg stood clearly silhouetted against a wash of stars and a waning crescent moon.

Now, just after 3 a.m., even the most prominent coastal landmarks have faded behind a fine mist, which creeps in stealthily, dropping condensation that curls the edges of the papers on my clipboard. Though I have been alone on Bolinas Ridge’s west slope since well before sunset, I’m actually “keeping company” to-night with nearly a dozen other people, including several of the finest field biol-ogists in the state. We’re using radio telemetry to observe the behavior of two of the park’s most elusive residents: Townsend’s big-eared bats and pallid bats.

Most folks would never know it, but the nine Bay Area counties are a veritable haven for bats. To join the ranks of bat-watchers, head to a favorite outdoor spot at sunset anytime between May and October. Visit Sunol Regional Wilderness, Tilden Regional Park, or Foothills Open Space Preserve. Stroll the landscaped campuses at Berkeley or Stanford. Take a seat on the beach at Bolinas, Pescadero, or Fort Funston. Occupy a bench beside one of the lakes in San Francisco’s Golden Gate Park or find an open spot in downtown Martinez. As the light dims, focus your attention on the horizon and scan the sky above the silhouetted crowns of trees. The movement around you will seem almost ghostly at first—just a flutter against the gloaming, a flash of streetlight glinting off sleek fur, a gentle ripple where wingtips, tiny feet, or mouth have just skimmed across the surface of a pond or stream. The shadows you see in flight, wherever you go, may be any of 14 species found in the Bay Area—from the ubiquitous little brown, big brown, or Mexican free-tailed bats, to the diminutive western pipistrelles and sparrow-sized hoary bats.

Bat Ways

If you prefer to keep your distance from bats, you’re not alone. Still, take a moment to reflect on this remarkable group of mammals, which includes almost 1,000 species worldwide and 24 in California. The top consumers of nocturnal insects, California’s bats contribute substantially to the control of mosquito, fly, moth, and beetle populations. Nectar- and pollen-feeding bats play the same role as bees and hummingbirds by pollinating a wide variety of night-blooming plants, from crop fruits to agave and saguaro cactus. Bats that consume fruit (none of these live in California) are seed dispersers essential to the health of rainforests. No human invention can replace these diverse ecological services, which bats have provided for at least 45 million years.

Over the past 150 years, as development has altered the California landscape, bats have faced the loss of roosting sites and the destruction of woodlands and waterways where they feed. Like birds, bats have been devastated by the use of pesticides that kill off their prey, contaminate water sources, and accumulate in their body tissues. Despite designation by the California Department of Fish and Game as a Species of Special Concern, Townsend’s big-eared and pallid bats—the species we’re studying around Point Reyes—are among the most threatened, victims of all these forces combined.

To make informed decisions about protection for these bats, our first job is to clarify their habitat needs—no small task, given that most species access three or more roosts over the course of a year, each roost with its own unique requirements. Winter hibernation sites must be free of disturbance and maintain a temperature at least a few degrees above freezing. Summer day or maternity roosts, used for sleeping and raising pups, should be fairly secluded, warm, and with a constant humidity. Depending on the species, these are found in caves, tree hollows or buildings; under rock crevices, tree bark, or house shingles; or among clumps of leaves. While feeding, bats access night roosts within their foraging territory (barns, porches, and the underside of bridges are common choices), where they can safely eat, nap, and socialize between meals.

Pallid bat
A pallid bat that was found and brought to the California BatConservation Fund for rehabilitation. Bats are banded after they arefully grown, males on the right forearm, females on the left. Photo byRobert Bloomberg/

It is now late September. The bats’ maternity season has concluded, and the pups, born in May and June, are well on their way to independence, making it safe for biologists to undertake some hands-on research. Yesterday, the research team captured a dozen or so big-eared bats and a handful of pallid bats emerging from their maternity roosts. Each was outfitted with the hippest of biological “jewelry”: a shiny, new one-gram radio transmitter, which was glued to each animal’s shoulder fur (bats being too small to wear bulky radio collars). Placed there, the tiny box and its antenna don’t interfere with the bat’s wingbeats and aren’t easily licked or bumped off. The transmitter’s battery runs for about two weeks, roughly the same amount of time it takes for the glue to peel away.

Tonight, as on each of our 10 allotted survey nights, research team members are dispersed far and wide around the park. Some of us remain, alone or in pairs, in one spot from dusk to dawn. Our job: find the bats by listening for their individual radio signals; document the location and direction of any movement; then pass the information along to mobile researchers, who dash from locale to locale trying to get close to each bat detected. Seemingly small details—whether the bat is stationary or in motion, in its day roost or holed up somewhere for a nighttime nap, feeding over water or some other habitat—are significant, because they tell us how the bats live out their night-to-night lives.

Like most of California’s 24 species of chiropterofauna (“hand-wing animals”), pallid and big-eared bats were described and assigned scientific names in the mid-1800s. Beyond physical descriptions, however, the lives of bats remained mysterious until radio telemetry, night-vision scopes, and sophisticated acoustics gear were developed in the mid-1900s. Biologists now know far more about how particular bat species spend their time in and out of the roost. We’re also discovering that social behavior, roosting and feeding habits, and seasonal activity patterns vary dramatically between bat species—and even within geographically-separate populations of the same species.

For example, while California’s inland Townsend’s big-eared bats commonly roost in caves and mines, coastal populations (having no access to caves) form maternity colonies in abandoned buildings and barns. When feeding, big-eared bats skim along the edge between forest and field or over stream corridors, hunting for small moths using echolocation. As they fly, the bats call out. By listening for the echoes produced when those calls strike solid objects, a bat can tell how large an obstacle is, whether it is moving or stationary, and a plethora of other details that aid in navigation and isolation of even the tiniest flying insects. Once located, prey is grabbed in the bat’s claws or trapped in the flap of skin that stretches between the bat’s legs, then passed to its mouth and consumed mid-flight.

Pallid bats take a different approach. While some choose buildings as day roosts, others opt for rock piles, or crevices and hollows in oak trees. Pallids echolocate as a means of navigation, but not generally for feeding; instead, they rely on their exceptionally acute hearing to detect the telltale movements of the big, ground-dwelling insects—such as Jerusalem crickets, Junebugs, and longhorn beetles—that are their prey. The insects are captured in the bat’s sturdy jaws, then brought back to the nearby night roost to be eaten.

Tracking #50

Our current research objectives are to study how members of the Point Reyes population spend the few remaining days (and nights) of summer, and to find out when they disperse toward winter roosts. So here I sit in the damp darkness, awaiting signs of bats in motion. Every 15 minutes I stand up, adjust the strap of my radio telemetry receiver so that the weighty black box rests against my ribcage, and flip it on. I calibrate the frequency dial to 574, then—pointing the T-shaped hand-held antenna halfway between the horizon and zenith—begin a slow clockwise circle starting at due north.

The female big-eared bat wearing transmitter 574 is nowhere to be heard. Resetting the dial to 529, I repeat the ritual, then again at 504. As I turn the antenna west, a series of loud pings suddenly peals out from the receiver. At the same time, rapidfire clicks erupt from my bat detector, proclaiming that 504 is flying close by. The detector is a clever little device that collects and records bats’ echolocation calls, usually undetectable by human ears. Though 504 sees fairly well (all “blind as a bat” myths aside), the big ears for which she was named—each about half her body length—are her real windows on the night-darkened world.

As I recite my coordinates into the walkie-talkie, a colleague announces, “504’s east of the cemetery!” One of the mobile teams is nearby on Highway 1; they, too, are following the bat’s movements. Over the next few minutes, everyone listens with rapt attention, taking readings and scribing data as the bat flies slowly but deliberately southeastward, finally disappearing over the top of Bolinas Ridge—in the opposite direction from her usual day roost.

Throughout the next day, we drive and hike the park’s back roads, attempting to locate signals for the few bats that never returned to the day roost. One signal is located in a barn; a quick trip reveals that the transmitter has fallen off, probably while the bat was using the building as a night roost. A couple of other signals are impossible to locate; these bats may have moved beyond receiver range, perhaps toward winter hibernation roosts.

We’re all especially fascinated by 504’s retreat from the group roost—her signal pings steadily on the east side of Bolinas Ridge, where there are no known colonies or even suitable buildings. Over the next few days, a concerted effort is made to discover where, exactly, she has holed up. Finally, the services of a local pilot and his twin-engine plane are called upon, and the location of the signal is isolated from the air. Coordinates in hand, a bat biologist scrabbles through the forest above Kent Lake and pinpoints the exact location of the bat’s roost. Leaning inside the hollow of a tree, he points his flashlight upward. It reflects on the shiny metal of an almost-new radio transmitter antenna and catches the coppery sheen of the fur on a snoozing Townsend’s big-eared bat—the first ever found to roost in a tree. One mystery solved—and another handful of questions raised. Is this the bat’s chosen place to sleep out the damp and mothless winter? If so, will other bats eventually join her here? Or is this just a way station en route to a more “permanent” winter roost? And might big-eared bats have used trees as day and maternity roosts in the era before buildings and mines became available, but old growth trees were abundant? Sadly, with our limited time frame for research drawing to a close, further investigation of these questions must wait until another year.

Inside the Old Barn

Three weeks later, I’m seated cross-legged on the attic floor of a century-old redwood barn at the Laufenburg Ranch, near the Sonoma-Napa county line. The air is already thick with heat, though it’s barely past nine o’clock on this October morning.

Thanks to the Sonoma Land Trust, which manages this lovely 175-acre spread, I’ve studied bats at Laufenburg Ranch over several consecutive summers. The ranch encompasses a mix of habitat types typical of the region: native oak savanna, lush riparian woodland, organic vegetable gardens and orchards leased by local farmers, and one of the most easterly redwood forests in the area. Consequently, it’s home to a terrific sample of Bay Area bat species.

The barn is a real treasure, a testament to the ways in which bats have learned to live with humans. At any point in the spring, summer, or fall, I can enter this barn and find evidence of several bat species. Today, there’s a hefty line of insect parts laid out directly below the attic’s high central beam. It’s unmistakably a pallid bat night roost, looking like a restaurant after the all-you-can-eat crab feast: beetle wing cases, antennae, and other inedible insect body parts are strewn everywhere. The pallid bats’ hard-shelled prey provides a bounty of meat, but getting to it requires some work. Hanging in the night roost, they peel off the insects’ hard exoskeletons, section by section, until the meat is accessible.

Climbing down from the attic, I give the barn’s main floor a final shine-over with my flashlight. A scattering of guano (hard fecal pellets) covers the seat of an old tractor in the storage room, where little brown bats like to sleep; dark oil stains below the tackroom’s loose window frame show where Mexican free-tailed bats have been squeezing in. In an old horse stall, I make a cursory inspection of the floor in search of fresh guano. Against the far wall, a few pieces rest atop a stack of rickety wooden packing crates. To my surprise, the tiny pellets have a golden sheen. Only one bat I know of leaves such a distinctive calling card: the Townsend’s big-eared bat, whose guano glitters with the undigested, iridescent scales of moth wings.

Not daring to move too fast, I drape my fingers over the flashlight to dim its glow, then point it upward. The small form hangs one foot from the ceiling, just to my left, and she (or he) is awake. We watch each other for a few minutes, the bat’s impressive two-inch ears—thin as paper—rotating and turning attentively to catch every nuance of sound in the dim room. Then, seemingly satisfied, the bat gives a stretching rustle of her wings and settles in for a nap.

Though the weather retains its summery edge, the days are noticeably shorter and the nights are cool. Bats know the signs and respond accordingly. They’re beginning to disperse, males and females seeking each other for their fall mating rituals, mothers and this year’s young separating to fly in search of cozy winter haunts. Many of our local species undertake migrations, flying a few miles—or a few hundred miles—in search of suitable hibernation roosts or warmer winter feeding grounds. Yet my hunch about the big-eared bat’s “ideal” hibernating site—a disturbance-free cave or mine that never freezes, located within 32 kilometers of the summer roost—is purely theoretical; I have no way of knowing where this bat will sleep out the winter. The Bay Area has no suitable caves, and few hibernating big-eared bats have ever been found here. Perhaps it is for the best that some mysteries remain intact.

Moving slowly, I back out of the room, shut the half-door quietly behind me, and step out into the October sunshine. These last encounters will keep me bat-dreaming through the winter months, until warm summer nights draw the bats, and me, out into their world again.

About the Author

Christine Petersen has researched bat ecology and behavior for the California Academy of Sciences, U.S. Forest Service, and U.S. Geological Survey over the past seven years. She also writes books on animal biology for Grolier Educational Publishing and teaches computer technology courses in grammar school.