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Bat Patrol

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Bat Patrol

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By Gary F. McCracken and John K. Westbrook

Photographs by Jay Dickman

Radar follows bats in Texas as they gorge on crop pests.

Read or print the full article.

The nightly flights in spring and early summer involve an estimated 100 million Mexican free-tailed bats emerging from a dozen major caves in south-central Texas. Perhaps another 50 million live in caves in Oklahoma, New Mexico, and Arizona. These bat populations are maternity colonies, huge congregations of females that migrate into the southwestern United States each spring from wintering grounds in Mexico. In June each female gives birth to a single pup. She leaves the cave twice each night to feed, returning to nurse the pup. By late July the young bats have grown enough to hunt with their mothers, doubling the number of bats flying from the caves.

To feed herself and her growing pup, a female bat must eat the equivalent of up to 70 percent of her body weight each night. Although each bat weighs only half an ounce or so, calculations show that a million bats can devour about ten tons of insects nightly. That means the 100 million free-tailed bats in south-central Texas must eat an incredible 1,000 tons—two million pounds (907,185 kilograms)—of insects in a single night.

Since bats in other locales pursue insects close to the ground, we wondered why the free-tails were flying as high as 10,000 feet (3,048 meters). Is this where they find the billions of insects needed to sustain their huge populations?

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Online Extra
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Doppler radar captures the nighttime chase of bats and bugs.

Bats make an evening exodus from their haven beneath a Texas bridge.
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Blame it on Bram Stoker’s Dracula—or a face only a mother could love—but bats send many into fits of terror. Why do bats conjure such fear? Do less-than-cute creatures get a bad rap? Share your thoughts.

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

When the 20 million female Mexican free-tailed bats in Bracken Cave give birth, their pups cluster close together, benefiting from the contact of many other warm bodies and reaching densities of up to 5,000 pups per square meter. It was once thought that when it came time to nurse the baby bats, female freetails would simply settle for whichever pup was most convenient. This was known as the milk-herd hypothesis. By matching the genes of nursing mothers and pups, however, scientists have found that in most instances—over 80 percent, in fact—a mother actually feeds her own young. Mother and pup each make use of the other's distinctive call in order to navigate toward one another in the darkness of the cave. As a final test before nursing, mom identifies her pup among the millions of lookalikes by recognizing its unique odor.

— Thomas Bruey

Bat Conservation International (BCI)
Find a wealth of information on bat species from around the world, as well as news about BCI's efforts to conserve the world's bats, ways to get involved, and extensive links to other bat-related resources.

National Weather Service Austin/San Antonio, Texas 
See the current view from the New Braunfels Doppler radar, and find links to more information about weather and Doppler radar from the U.S. National Weather Service.

Bat Population Database
Look up information about bat populations throughout the U.S. with this database, maintained by the United States Geological Survey, that lets you search by state, territory, or species.

Insect Identification from Texas A&M University's Department of Entomology
Learn about the hundreds of insects that are common in Texas and elsewhere, including the corn earworm and other pests, with helpful background information and photos. 

U.S. Department of Agriculture
Find a number of links at this site to programs and sources of information on agriculture in the United States, from statistics to agricultural policy to research initiatives.


Berenbaum, May R. Bugs in the System: Insects and Their Impact on Human Affairs. Addison-Wesley, 1995.

Fenton, M. Brock. Bats. Checkmark Books, 2001.

Metcalf, Robert L., and Robert A. Metcalf. Destructive and Useful Insects: Their Habits and Control. McGraw-Hill Inc., 1993.

Nowak, Ronald M. Walker's Bats of the World. Johns Hopkins University Press, 1994.

Tuttle, Merlin D. America's Neighborhood Bats. University of Texas Press, 1988.


Tuttle, Merlin D. "Saving North America's Beleaguered Bats," National Geographic (August 1995), 36-57.

Sajewski, Lise. "Upside Down or Bat Side Up?" National Geographic World (October 1995), 7-11.

Tuttle, Merlin D. "Bats--The Cactus Connection," National Geographic (June 1991), 130-140.

Tuttle, Merlin D. "Gentle Fliers of the African Night," National Geographic (April 1986), 540-558.

Tuttle, Merlin D. "The Amazing Frog-Eating Bat," National Geographic (January 1982), 78-91.

Novick, Alvin. "Bats Aren't All Bad," National Geographic (May 1973), 614-637.

"Bats!," National Geographic World (October 1975), 12-15.

McCue, J. J. G. "How Bats Hunt With Sound," National Geographic (April 1961), 570-578.

Griffin, Donald R. "Mystery Mammals of the Twilight," National Geographic (July 1946), 117-134.

Bailey, Vernon. "Bats of the Carlsbad Cavern," National Geographic (September 1925), 320-330.


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