At dusk under the I-35 bridge at Camden Street a small crowd lingers. They bob their heads between wristwatches and cell phones and something above not yet visible. Passersby stop to ask, “What’s everyone looking at?”
Soon a cloud of Mexican free-tailed bats are streaming from beneath the bridge, the façade of the San Antonio Museum of Art fading in the twilight. The bats are coming out to feed. “A colony of approximately 10,000 Mexican free-tailed bats visit this bridge every year,” says Matthew Driffill, an education specialist with the San Antonio River Authority, pleased with the growing interest. “Nature is everywhere, even underneath I-35, one of the busiest highways in San Antonio.”
For the second summer in a row, Driffill and Matt Reidy, a biologist with Texas Parks and Wildlife, are coordinating bat tours — though the colony typically arrives from Mexico in March and doesn’t fly out until October. The pair educates the public about the benefits of bats and works to dispel any lingering myths about the often-misunderstood creatures. For instance, rabies in bats is exaggerated, found about as frequently as it is in dogs, according to John Gramieri, mammal curator at the San Antonio Zoo. “But if you find a bat on the ground, its health is probably compromised. You don’t want to interact with it,” he said.
The talks drew about 300 in July of 2010, but the pair expects larger crowds this summer as activity on the Museum Reach of the River Walk increases and word spreads about the colony. They caution, however, that the bats aren’t on their payroll and don’t always emerge 5-15 minutes before sunset as is their usual schedule. “Biologists have theories about the conditions under which bats emerge,” says Susan Kwasniak, a spokesperson for Austin-based Bat Conservation International (BCI), an organization of biologists and educators who research the flying mammals’ role in the environment. “It’s dependent on the amount of insects in an area. If there is a drought, the bats may emerge earlier to allow more time to find food. If it rains, they may come out later because rain is conducive to insects and it’s easier for them to find food. Also, as it gets darker, the bats are safer from predators like hawks.”
Bats fly up to 50 miles in search of insects, mostly cotton bullworm and corn earworm moths, a service the BCI estimates saves Texas cotton farmers $1.7 million a year by reducing the amount of pesticides needed. Texas is home to the world’s largest known urban bat colony (the Congress Bridge in Austin with 1.5 million bats) and the largest rural colony (Bracken Cave in southern Comal County, with 20 million bats).
Yes. The world.
Mexican free-tailed bats adjust easily to human-engineered environments, and TxDOT’s highway expansion joints beneath I-35 and the Congress Street Bridge are just the right size to provide shelter while protecting the bats from predators, according to Kwasniak. And armed with knowledge of their economic contribution to the state, the agency is now actively choosing bridge designs that can double as bat roosts and nurseries.
Historically, bat colonies in bridges atop the San Antonio River were removed to prevent exposing people — not to mention Rio San Antonio cruise and dinner barges — from falling guano, Driffill said. However, the colony under the I-35 bridge over Camden is advantageous for all. “We have a great situation at this bridge. The I-beam structure is above the waterway. The bats, however, live on the other side in the expansion joint. All the guano then is in an area well removed from the river,” he said. •
The San Antonio River Authority will host bat talks on the Museum Reach at Camden and Newell Streets on July 12 and 26 at 8pm, and Aug 9 and 23 at 7:30pm. Call (210) 302-3222 for more information.
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