Bats Get Custom Homes as Natural Roosts Vanish

Brian Handwerk
for National Geographic News
October 27, 2006
Walk near the Congress Avenue Bridge in Austin, Texas, on a summer
evening, and you'll be tempted by bat T-shirts, bat photographs, and bat
paraphernalia in all shades of black. Nearby hotels tout rooms for their
outstanding bat views.

It's all part of an industry that has sprung up around the city's nightly bat emergence, when an awe-inspiring colony of some 1.5 million Mexican free-tailed bats flee their bridge roost each evening at dusk.

The spectacle began after the bridge was renovated in the 1980s to include deep, narrow crevices on its underside that were unintentionally ideal for bat nesting.

Today the bridge draws tens of thousands of tourists when the bats are in residence (from March to October) and contributes an estimated eight million U.S. dollars to the local economy.

"It has really become integrated into the culture," said Barbara French, biologist with Bat Conservation International (BCI) in Austin.

"Even the hockey team is named the Ice Bats."

The bridge is a unique attraction, but it's also a blueprint for ongoing efforts to convert bridges, highway culverts, and other suitable locales into new bat habitat.

"One of the ways that people are working to mitigate the loss of natural habitat is through the development of artificial roosts," French said.

Same Bat Station?

The Texas Department of Transportation currently outfits dozens of bat-friendly bridges per year.

"Particular bridges have expansion joints in the bottom of the bridge that are about three-quarters of an inch [1.9 centimeters] wide and 12 inches [30.5 centimeters] deep," said Meg Goodman, bat biologist with the Texas Parks and Wildlife Department (TPWD).

"These crevices are just the right size for bats."

Bridges aren't the only artificial bat habitat that has proven appealing to the flying mammals.

Bats were among the unheralded victims of recent Gulf Coast hurricanes.

So many bottomland hardwood trees, where many bats roost, were destroyed by those storms that volunteers have begun constructing cinderblock structures in eastern Texas's Trinity River National Wildlife Refuge.

The towers mimic hollow trees and seem to be tempting many roosting bats.

Why all this effort to find bats a new home?

Biologists stress that bats provide many ecological benefits and that their bad reputation is largely a matter of misinformation.

Bloodsucking Bats Real But Rare

Despite their legendary association with the undead, only one bat species out of more than a thousand—the vampire bat of Central and South America—feeds on mammal blood.

Vampire bats typically target cattle or horses. Human bites from vampires—or any other bats for that matter—are rare.

A Houston teenager died of bat rabies in May, but only 48 confirmed cases of rabies from bats have occurred in the U.S. in the past 55 years, according to BCI records. Bat lovers argue that humans should focus on the flying mammals' largely positive impacts.

They pollinate plants and produce an excrement—guano—which farmers esteem as fertilizer.

Bats also travel dozens of miles each evening to devour moths, mosquitoes, and other insects in impressive quantities.

The Mexican free-tailed bat, for example, eats two-thirds of its weight each night.

Because of their voracious appetites, bats function as extremely effective and pesticide-free pest control.

Among the Mexican free-tailed's favorite meals is the corn earworm, a crop-raiding pest that savages cornfields from Texas to Canada.

"Corn and cotton farmers spend millions spraying for these particular pests, and research finds that the bats save at least two applications per year," TPWD's Goodman said.

Meanwhile, Back at the Batcave

Mexican researchers with the Technological Institute of Victoria City in Monterrey have found alarming bat declines in some important cave colonies, at rates up to 90 percent.

Many species live in Mexico during colder months and in the U.S. Southwest during warm seasons, so declines could impact numbers on both sides of the border.

Diminishing habitat, like fewer suitable cave and roosting sites, has had a negative impact on bat populations.

Bats typically have only one pup per year, an unusually low number for mammals of their size. The slow reproductive rate means that decimated populations may have a tough time bouncing back.

Artificial habitats can be helpful, but protecting traditional roosting sites may be the most critical step conservationists can take.

BCI owns Bracken Cave, near San Antonio—summer home to some 20 million migrating bats, making it the world's largest bat colony.

Last month the Mexican environmental organization Pronatura Noreste purchased a heavily populated cave near Monterrey and made it a conservation site to help spare it from the pressures of tourism and pollution.

The Cueva de la Boca cost the group some $500,000 (U.S.), but the organization considers the sum a wise investment in bat futures.

"When you're talking about large bat colonies that roost in a small number of caves, that's a problem," BCI's French explained.

"If you have a closure or disturbance of a single cave you could destroy literally millions of bats in a single incident," she said.

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