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A Night at Capitol Lake

It is summer 2008, the year Bats About Our Town was founded. The time is twenty minutes before a late July sunset at Capitol Lake in the center of downtown Olympia. The calm lake surface reflects the state capitol dome and the air gradually cools after a sunny day.

A group has gathered for a bat walk with biologist Greg Falxa of Cascadia Research Collective. I’m standing along the Deschutes Parkway near lamppost 39, a favorite lookout point for the early-arriving Big Brown Bats.  My back to the lake, I’m looking up at the ridge above me.  Huge trees stamp black filigree silhouettes against a sky darkening to deep purple-blue.

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Big Brown Bats: The First Arrivals 

I’m concentrating on the spaces between the trees, keeping my eyes in soft focus so I can see a wide panorama. Watching as the bats arrive is like watching for shooting stars, silent and fast.  One moment, a flutter at the top of the ridge; the next moment, a form just above my head, plummeting down to the waiting lake. 

The Big Brown Bats are not silent, of course.  I just can’t hear their calls, which actually are so loud they might be called shouts.  They are flying open-mouthed, pulsing out their high frequency calls. Like whales and like submarines, bats can navigate using sound.  The bat that just flew by my shoulder knew exactly where I was—she saw me with her eyes, and she heard the echoes of her calls bouncing off me.  Her sonar works up to a 30 foot distance.  Beyond that, she must use her eyesight alone.

Hunting for Insects

Now this bat dodges over the lake and into the shrubby margins, hunting. She is looking for small flies, small moths, midges, and other insects.  Like all the other bats soon to arrive at the lake, she may catch an insect in her mouth, but she has another option.  She can scoop her prey into the membranes of her wing or her tail.  Then she reaches down into the membrane pocket, supple as my cat licking her hind leg, and eats the insect.  This can happen several times a minute.  After eating, she may change directions, doubling back for another nearby insect.  With the eating and the doubling back, her flight can look clumsy and fluttering to me, when in fact it is extraordinarily efficient.

Yuma Bat Catching a Moth

(Click on the image to see a larger image)
Photo © Merlin Tuttle

The bats aren’t just scooping up insects.  They scoop the air as well with their webbed wings, just as a duck’s feet move it through water.  Bats are the only mammals that can fly. Our fantasy angels have wings on their backs, but bats row the air with their hands.

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Time to Use the Bat Detectors

Quickly it becomes too dark for me to see well with my eyes.  Yes, I notice the bats crossing the lamplight ringing the lake—if I’m lucky.  They fly very fast.  Now I do as they do, “seeing” with my ears. I have an electronic device called a bat detector.  Mine is about the size of an old-fashioned telephone handle.  The detector ramps down the bat calls into sounds I can hear.  When the sounds speed up, a series of harsh rapid clicks, the bat is closing in on an insect.

Bat Detector

New species of bats are arriving now, half an hour after sunset. As a beginner with simple equipment, I can’t identify the bat species on the spot.  Greg Falxa, with his researcher’s equipment, his researcher’s honed ear, and his many nights’ experience at the lake, gives us his best guess about what is flying by.  (The recorder attached to his equipment can be used to verify all this later—and indeed my detector can be attached to a recorder too for later analysis.)

California and Silver-Haired Bats

Greg tells us that the smallest bat in our area, the California Bat, may be flying by.  These tiny bats drink water from the lake but feed along the lake edge among the shrubs, and up in the tree canopy near the lake.  They find the midges, tiny like themselves.

Now we hear the calls of a passing Silver-Haired Bat.  This densely furred bat emits a flat, even call about three times a second.  Relatively uncommon, these bats are solitary or roost in small groups, and live in trees.

Yuma and Little Brown Bats Will Come in Groups

We are eagerly awaiting the main event of the night, the arrival of thousands of small Yuma and Little Brown Bats, customers at the abundant Capitol Lake cafeteria since late spring.  These are mother bats who live in large maternity colonies.  Roosting together helps them conserve body heat.  Now their pups, born a bit late because spring 2008 was very cold, huddle together at the roosts while the mothers come to Capitol Lake for food.  The mother bats emerge about fifteen minutes after the sun goes down, resetting their internal clocks every day.

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Three Known Bat Flyways

Unlike the other bats we have seen so far, these mother bats don’t just wander over to the lake. They come in groups, following regular flyways that have been in existence for many years. At this point they are navigating by sight as well as by echolocation. While flying above the tree canopy they use tall trees that reach above the canopy as navigation points, passing one tall tree and then another.  Their path is quite predictable, and if you know which are the navigation trees, you can watch the bats stream by after sunset. You also can watch them follow a particular street for so many blocks, go single file down a specific alley, and veer left around that big fir tree. 

We know of three bat flyways coming in from the north and west. 

  1. The flyway starting in The Evergreen State College area sends 600-1000 bats to the lake.  They start arriving at the lake 30 minutes after sundown, with the peak at 45 minutes, and the end about an hour after sundown.
  2. The Cooper Point/Eld Inlet/Totten Inlet flyway sends about 1000 bats, arriving a little later than the Evergreen group.
  3. From Woodard Bay in the North Olympia area, 3000 mother bats are taking half an hour to make their 8 to 10 mile trip to the lake—16 to 20 miles round trip.  This is the largest known commute in North America for Yuma and Little Brown Bats, weighing about what a nickel weighs.  While nursing their pups, the bats often make the trip twice a night.

Greg Falxa’s research shows that the mother bats leaving the lake probably don’t mix from one of the three social groups to another group. Each goes home to her specific roost and her waiting pup.  Quite possibly, other bat flyways reach the lake from the south, arriving from using volunteers acting as bat detectives (to learn more, see the volunteer form under How To Help).

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Yuma and Little Brown Bats Arrive

We are walking north along the lake.  The bat detectors begin to talk as the hungry mother Yuma and Little Brown Bats arrive, flying in above the West Side of Olympia.  Occasionally we see a rapid flicker as a bat plummets down by a street lamp on her descent to the lake.

Some of us contine north, crossing below the Fifth and Fourth Avenue bridges to arrive at the boardwalk by the Bayview Thriftway grocery store.  Many of the far-flung Woodard Bay bats fly in under the boardwalk until they reach the riprap boulders along the shoreline by the east end of the Fourth Avenue bridge.  Then they dodge under both bridges, hugging the sides, emerging at last at the lake after half an hour’s journey.

Video Camera Helps Us See Bats

The lake now is embroidered everywhere by tiny bats feeding on the midges and other insects that hatch regularly throughout the summer, providing a rich, reliable food source.  We cannot see the bats, though. We turn back to join the group at lamppost 43, where a video camera feeds images into a TV monitor sitting on a card table.   The camera points at the bright rectangle of light cast on the lake surface by a hotel east of the lake.  We can see constantly moving bats, almost as dense as the swarms of midges they enjoy eating.  Some people are watching with binoculars trained on the pool of light.

Books about bats may talk about bats taking a rest during their night feeding.   Some of Greg Falxa’s Yuma and Little Brown Bats wearing radio transmitters did no such thing.  They fed for up to six hours straight before heading home—and these research bats all came from Woodard Bay, a good hour’s round trip commute away, amounting to seven hours of constant flight.

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Midnight at the Lake

It’s midnight.  The night air, seemingly silent except for the hum of a passing car, in fact is vibrating with high-pitched sound.  Thousands of bats continue their stuttering flights above the water, pausing several times a minute as they catch midges, caddis flies, an occasional moth.  Each bat has her preferred section of the lake where she spends most of her time hunting, thus helping distribute the bats evenly above the water’s surface.

A Townsend’s Big-Eared Bat makes a cameo appearance by the Interpretive Center at the south end of Capitol Lake.  Extremely rare, on the endangered species list in fact, this bat hunts moths.  In the dance between predator and prey, moths have evolved very sensitive hearing—and these bats have evolved very soft echolocation calls, hence their huge ears.

Two AM

A light summer rain peppers the lake.  Bats come and take refuge under the Percival Cove bridge, where they hang out, so to speak, waiting for the rain to stop.  The corrugated surface under the bridge is like an egg carton, an ideal place for bats.   After a few minutes of no rain, the bats charge back over the lake to resume feeding. Their calls, inaudible to humans, are in fact extremely loud—think of five car thousand horns honking.  Bats have evolved to cope with all this noise.  Each bat has a specialized hearing apparatus that filters out most other bats’ calls before they reach the bat’s brain.

Half an Hour Before Sunrise

Final departures from the lake are beginning. All night bats have been coming and going.  Woodard Bay bats while nursing their pups may make two trips to the lake, for example.  Typically end their first feeding between midnight and 2 AM, go home, then return for another meal.  Only the stragglers are left at half an hour before sunrise, and they hurry home.  As the sun rises behind the Eastside Olympia hill, as humans wake and start their day, the bats and their pups are tucked away in cracks and crevices all around Thurston County, starting their long daytime sleep.

Visit Capitol Lake for the history and future prospects of the lake.  

Create Your Own Bat Walk at the Lake

  • Bring a pair of binoculars with good light-gathering capacity.  
  • Arrive ten minutes before sunset (for the guided tour people come earlier in order to hear an interpretive talk)
  • Position yourself at lamppost 39 along the Deschutes Parkway, near the bus stop shelter halfway between the 4th and 5th Avenue Bridges and Marathon Park.  Look west up at the ridge to see Big Brown Bats arriving.
  • As the light fades, walk north along the lake, noticing any bats that zoom past the lampposts as they approach the lake.
  • Cross Fifth and Fourth Avenues to arrive at the walkway by Bayview Thriftway that terminates at the Fourth Avenue Bridge, and look for bats flying low along the boulders and then under the bridge.
  • Return to the Deschutes Parkway, going to lamppost 43.  To see bats hunting insects, train your binoculars on the large rectangle of light cast by the hotel.

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