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What Bridges are Teaching Us About Bats
It’s a lovely spring evening in Austin, Texas. A crowd gathers on the Congress Avenue Bridge but not to enjoy the sunset. Instead they are there to watch the nightly emergence of the largest urban bat colony in North America - up to 1.5 million of them! The bats weren’t always so popular. They were once considered pests at best; dangerous at worst. But thanks to conservation groups’ efforts to educate the public about this much maligned creature, the city has turned what was once considered a liability into an asset, to the tune of as much as $10 million a year in eco-tourism including bat-watching boat trips and restaurants who advertise their views of the nightly spectacle! A trip to your local gardening specialty store is also likely to turn up bat “houses” these days. Gardeners and farmers are beginning to appreciate the role bats play in controlling pests and the diseases they may carry, as well as a number of insects that damage crops. And while most bats found in the US are very small, they can eat more than 50% of their weight in insects each night. According to the Center for North American Bat Research and Conservation in “Bats of Indiana”:
According to the Kentucky Department of Fish and Wildlife:
As development spreads, however, these useful creatures are being pushed out of their traditional roosts and are looking for new homes. A survey of 20+ states showed that about 1% of bridges, like the Congress Avenue Bridge, provide the appropriate conditions to serve as man-made roosts. One such bridge in Southwestern Indiana has become invaluable in unveiling new information about the endangered Indiana bat, a species whose life history is still mostly a mystery.
Since 1967, the US Fish & Wildlife Service (USFWS) has listed the Indiana bat as an endangered species. Caves and summer forested habitat is especially important for this mammal, whose head and body measure only two inches and whose adult weight equals roughly that of a quarter. These tiny creatures are considered particularly vulnerable because they hibernate during winter in densely packed groups in only a few caves. For years, scientists have studied bats at these winter sanctuaries. They’ve watched swarms of bats entering and leaving caves, capturing and banding several. Scientists knew the bats were out and about the rest of the year, spreading out during the summer to live in trees, structures and bridges in smaller colonies, having their offspring, and then migrating back to their winter homes. But they didn’t know as much about their migration patterns, such as whether Indiana and other bats made stops under bridges, or if they returned to certain trees, structures, bridges and caves. New research done by BLA and the Indiana State University (ISU) Center for North American Bat Research and Conservation and funded by the Indiana Department of Transportation (INDOT) and Federal Highway Administration (FHWA) may reveal the answers to these and other questions.
Indiana bats were first found under a bridge in 2004 when BLA managed the inspection of 259 bridges and culverts in Southwestern Indiana for the presence of bats as part of its environmental studies for the proposed I-69 project. Inspectors found Indiana bats under only one bridge, located some distance from the final I-69 route. The USFWS asked that the location of the bridge be kept confidential to discourage human visitors, since graffiti indicated people have used the area under the bridge. As a result, INDOT and FHWA paid for fencing and locked gates to be installed to block access under the bridge. After the initial discovery, BLA biologists returned to the bridge regularly from October 2006 to December 2007. During this time, BLA biologists studied bats under this bridge with help from USFWS and ISU staff, as well as from Environmental Solutions and Innovations (ESI), who installed temperature data loggers under the bridge. This was the first in a series of studies that helped establish baseline data for resident and migratory bats. In 2008, BLA and ISU were given approval by the USFWS to band bats, including the Indiana bat, and examine them for signs of “white-nose syndrome,” a mysterious disease that’s devastating bat populations in the northeastern US and contributing to the species remaining on the federal endangered species list. Luckily there have been no signs of white-nose syndrome in Indiana yet. During the study, funded by INDOT and FHWA, researchers found the bridge was used by a community of bats: principally Indiana and little brown bats, but also a few big brown bats, eastern pipistrelles, and a single gray bat, another federally endangered species. The study also revealed that the structure served as a major migratory stopover for bats, especially the Indiana and little brown bat. Since little is known about the migration of bats outside of caves and mines, the bridge is providing exciting new insights to their behavior including evidence that Indiana bats emerge earlier in the spring and stay out later into the fall than previously thought. One male bat was even found roosting under the bridge into early December.
Moreover, while pregnant female bats have used bridges as night roosts during regular summer feeding bouts, possibly to use the heat retained in the concrete to stay warm and conserve energy, it appears that males may use bridges for mating and may spend more time there than the females. Typically male Indiana bats fly to caves in the fall and congregate at the mouth of the caves waiting for females to arrive to mate. But scientists found more males under the bridge in fall indicating that they may be waiting to mate with females just like at the caves. This means the bridge not only serves as shelter from predators and the elements, but also provides a place for bats to mate and roost, as well as serving as a rest-stop during migration. ISU is currently compiling a database of information from Indiana bats caught and banded by BLA, ISU and others to map migration patterns in the Midwest as well as to monitor for signs of white-nose syndrome. This information will contribute to the knowledge needed to stabilize the Indiana bat population and eventually move them off the endangered species list. “We’ve known very little about migration and wondered about ways to study it,” said John O. Whitaker, Jr., PhD, Professor of Biology at ISU. We are “getting a huge amount of data in a very localized area from the bridge study and that’s really going to add to the base of knowledge.” If you would like to learn more about BLA’s environmental services, please contact Dr. Tom Cervone at 1.800.423.7411 Resource links: |
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