Environmental Review Toolkit
Water, Wetlands, and Wildlife

Wildlife and Habitat

Attention Motorists - The Bats have Landed on Our Bridge

Although highway agencies always consider wildlife when designing and building highways, we don't often associate impacts on bats with highway structures or construction. This is in spite of the fact that bats are among the most diverse group of mammals, and at one time the most numerous and widely distributed. And, if we are to believe new scientific information on the activities of bats, one of the most important and beneficial. Yet bats are often misunderstood, sometimes even feared, because of a lack of knowledge about their life history and habits. These misunderstandings are perpetuated by the connection of bats with such unsavory mythical characters as Dracula.

When it was first discovered in the early 1980's that Mexican freetail bats had established a roosting and breeding colony between the beams of the rehabilitated Congress Avenue Bridge in downtown Austin, Texas, there was immediate public concern about public health and safety. Many people were concerned about the spread of rabies and other diseases by bats to humans and animals. Through the educational efforts of Bat Conservation International, a private conservation and educational organization whose headquarters are located in Austin, the initial concern has been turned into civic pride, and the bat colony has become a local tourist and recreational attraction. Largely through the work of Dr. Merlin Tuttle, world-renowned bat biologist and heat of Bat Conservation International, the Congress Avenue Bridge bat colony is now know as the largest urban colony of bats in the world, with a population estimated at one and a half million, and is an outstanding example of melding transportation facilities, public recreation and ecological needs.

Families often come to the shores of Town Lake, adjacent to the bridge, to picnic and observe the bats on summer evenings, when they become active and fly out from beneath the bridge to disperse in the surrounding countryside and feed on insects. The city of Austin has adopted the bat colony, and publicizes this unique ecological and educational resource as a tourist attraction to be enjoyed and appreciated by everyone. Children especially enjoy the sight of thousands of bats leaving the bridge in the evening, flying in such close formation it would seem that collisions are inevitable. Some enterprising Austin entrepreneurs have even printed up bat t-shirts!

Mark Bloschock, a bridge engineer with Texas Department of Transportation (TxDOT), became interested in bat ecology when he found out that bats were using the narrow crevices between the bridge beams as places to roost and rear young. Mark's initial concern was much the same as that of the public - were the bats a hazard to public health? He was also concerned that the potentially corrosive effects of bat guano, which is accumulated bat feces, would be detrimental to the bridge structure itself. Mark first established a research program to determine the impacts of bat colonies on bridge structures. These investigations have shown that, so far at least, there have been no ill effects from bat colonies on concrete or other structural elements. And as a result of Mark's collaboration with Bat Conservation International, TxDOT now has a research program studying the roosting preferences of different species of bats found in Texas. The results of the research will allow TxDOT and Bat Conservation International to develop roosting sites for bats in other highway structures, such as culverts and bridges, where that is desirable or needed to enhance bat habitat, or to exclude them where their presence is unwanted.

Dr. Tuttle and other biologists working on the life histories and habits of bats have shown that bats are rarely responsible for transmitting rabies and other infectious diseases to humans, and are not generally a cause for concern about human health and well-being, except when they are not present! Bats are exceptionally resistant to many animal diseases, and although just like other mammals are capable of contracting and transmitting rabies, they are remarkably non-aggressive. To avoid contracting rabies or other diseases from bats, just don't pick them up or otherwise handle them. They are also remarkably clean and free of external parasites, probably because they groom themselves carefully to make maximum use of their flying ability.

The precision and accuracy of bats' navigation systems is such that collisions between flying bats rarely occur, even in very tight quarters. I observed this amazing ability in the attic of a historic church in Pennsylvania, where I stood among a colony of 10,000 little brown bats and watched them fly around the small space and never collide with one another, the walls or columns, or my hair.

In many parts of the world, including the United States, bat populations are in precipitous decline. Part of this loss is due to losses of roosting sites and feeding habitat. In some parts of the world, bats are considered culinary delicacies, and are hunted and captured for food. In a few areas, such as Latin America, they are truly pests, primarily the blood-feeding species which have become known as vampires. These blood-feeding bats are sometimes a source of economic losses in cattle and livestock. Chemical analysis of their saliva has resulted in the discovery of a new anticoagulant drug which is now being tested as a treatment for human heart disease.

Bats usually live in large, centralized colonies, in some cases numbering in the millions of individuals. The size of the bat colony at Eagle Creek Cave, Arizona, was estimated at over 30 million bats at its peak in the 1960's. This colonial life style is in part because suitable roosting and breeding sites, such as large caves, are not widely distributed, and bats are capable of traveling great distances to feed and still return back to a roost. Another advantage is that during cold seasons, the bats can share body heat, and by staying close together, they can regulate their environment and use their fat supplies more efficiently. They are very tolerant of each other, and through unique behavior and adaptation, can successfully use very small spaces and crowd a lot of bats into a small area. Most bats, remember, are very small animals, although a few species such as the flying fox of Madagascar have a wingspan of up to six feet.

Bats have varied feeding habits - some are insectivorous, consuming several times their own weight per day in insects. The Eagle Creek Cave bat colony in Arizona was estimated to consume up to 200 tons of insect crop pests nightly. In Texas, the Bracken Creek Cave colony of over 20 million Mexican freetail bats are estimated to consume about 250 tons of insects, mostly crop pests and mosquitoes, nightly. Other species are nectivorous, feeding on flower pollen and nectar. These bats are important in the cross pollination and fertilization of plants over a large part of the world. They are especially important in the Sonoran desert of the Southwestern United States, where they are critical to the pollination and seed dispersion of many species of cactus and other desert plants, and in rain forests, where they are the primary source of seed dispersion and revegetation. Over 400 species of commercially-important plants are known to depend on bats for pollination and seed dispersal.

A few of the larger species are carnivorous, feeding on small mammals and even fish. Bats feed on insects and larger prey through unique adaptations of their unique and well-known echo location system, which allows them to locate prey and objects by projecting high frequency sound waves and locating the source of the echo. The accuracy and speed of this system of navigation and location is such that bats can locate and capture flying insects, small mammals, and even fish near the surface of small ponds by "reading" the patterns of ripples in the water.

Some, but not all, bats are true hibernators, their body temperature dropping to nearly that of their surroundings, but they are easily aroused by warming. When disturbed too frequently during hibernation, they use up the critical supplies of body fat they require to live through this period, and are unable to survive until food is available.

North American bat species are almost completely beneficial through controlling insects and pollinating plants. Of the forty-four species native to North America, six are officially listed as endangered and eighteen more are candidates for listing under the federal Endangered Species Act. The major cause of the declines in our bat populations is thought to be primarily careless destruction by human activities, including deforestation and cave or mine closures. In addition to the research program with TXDOT, Bat Conservation International is working with federal agencies such as the Bureau of Land Management, Bureau of Mines, Fish and Wildlife Service, and others to enhance bat habitat wherever possible, often through installing protective "gates" over caves or mines used as colonial roosting and breeding sites. The Texas Department of Transportation deserves our support and praise for initiating a thoughtful and unique research program which should benefit bats throughout their natural range, and at the same time benefit the ecosystems with which they share a mutual dependency. Mark Bloschock and Bat Conservation International hope that their research and habitat improvement program will benefit both bats and people.

Those wanting more information on the ecology of bats, their importance in ecosystems around the world, and measures which can be employed to conserve and enhance habitat for these beneficial animals (especially if you need some insect control!) can contact Bat Conservation International at the following address:

Bat Conservation International
P. O. Box 162603
Austin Texas 78716

Questions and feedback should be directed to Dan Buford (Daniel.Buford@dot.gov, 202-366-8168).

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