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Wildlife and Bridges
Photo: BridgeBridges obviously serve a very important role in our ability to traverse over lakes, streams, rivers, roads, railroads, the Chesapeake Bay, and a multitude of other obstacles. In our travels we typically roll over the bridges and give little thought to what might be residing beneath them. But did you know that bridges are important for wildlife too? Some of you may be familiar with the highly publicized Congress Avenue Bridge in Austin, Texas [batcon.org], where 1.5 million Mexican free-tailed bats and their young roost. Or perhaps you have seen osprey or gulls nesting on bridges. But you may be surprised to know that there are a number of different species that have adopted bridges as a place to rest, a place to feed, or a place to raise their young.

Because bridges have become a "habitat structure" used by wildlife, the Virginia Department of Game and Inland Fisheries has begun documenting where and how wildlife are using bridges in Virginia. What started out as "opportunistic sightings" has turned into a full-blown "Wildlife and Bridges" project. VDGIF is working with Dr. Bill McShea of the Wildlife Conservation Society and the Virginia Department of Transportation to better understand the types of bridges and associated habitats that draw wildlife to them. Below is a preliminary overview of some of our findings and a look at the future direction of the project.

Who's Using the Bridges?

Besides you and I, we've found seven species of birds, eight species of mammals, and one reptile that use bridges. The birds include barn, rough-winged, and cliff swallows; rock dove; eastern phoebe; osprey; and peregrine falcon. The mammals include Rafinesquee's big-eared, little brown, big brown, eastern pipistrelle, gray, and northern long-eared bats; woodrats; and gray fox. The only reptile that we've found so far was a black rat snake that was feeding on a bat. This gives us a total of 16 different species of wildlife utilizing bridges so far. As the project progresses we expect to find that more species use bridges.

Photo: Guano pile below expansion joint on bridgeHow Many Animals Do You Find at a Bridge?

While Virginia doesn't have a situation like that at the Congress Avenue Bridge in Austin, we do have bridges that contain sizable wildlife populations. The actual number of individuals varies depending on the species. Peregrine falcons, for example, are highly territorial and will not tolerate another pair in their vicinity. Therefore, you could only expect one pair of peregrines at a bridge. However, bats and swallows are colonial breeders; finding 20 to 50 cliff swallow nests or 100's or even 1,000's of breeding bats would not be unusual. With watchable wildlife becoming a popular pastime, we hope to identify bridges that would be of interest to the naturalist.

In What Parts of the State Are We Finding Them?

We've looked at bridges throughout the state and found that wildlife uses bridges across all of Virginia. However, when we looked at bridge use in the three major physiographic regions (coastal plain, piedmont, and mountains) we found some differences in bridge use among these regions. The coastal plain has shown the least amount of use so far with only 18.4% of the bridges being occupied by four species of birds and one species of mammal. The mountain region follows with 34.8% of the bridges being occupied by five species of birds and seven species of mammals. The Piedmont Region is the big winner with 50% of the bridges being occupied by five species of birds, three species of mammals, and one reptile. These are preliminary data and as we visit additional bridges in each of the physiographic regions these numbers will likely change.

Photo: Rafinesquee's big-eared bat.  Courtesy Bat Conservation International.Why Are They Using Bridges?

I'm glad you asked this question. What we need to keep in mind is that some animals are opportunistic and will adapt to changes in their habitat. For example a stream bank may have been lost or converted due to flooding, vegetation growth, or human development (rip-rap). In place of the stream bank, the tall bridge over the river may provide a niche for rough-winged swallows to make their nest. A small bridge with I-beams over a creek may provide a ledge for a phoebe to make a nest. The expansion joints in an overpass may provide a tight secure crevice for bats to raise their young. The large rock boulders below a bridge may provide a den site for a gray fox. Or the barn swallow nesting on the bridge may provide an easy meal for a hungry black rat snake. As with many man-made structures, wildlife can find a use for our technology as well.

What Habitats Around Bridges Do Wildlife Prefer?

Initially we only looked at the habitat directly under the bridges ("local habitat"). However, Dr. McShea and his team will be looking at the surrounding habitat around bridges to see if the make-up of habitats at the landscape level affects bridge use. With our first cut at "local habitat" use we found that we sampled bridges over streams and rivers more often than bridges over any other habitat type (lakes, railroads, roads, etc.). Despite this bias we saw some interesting patterns. First, our initial thought or hypothesis is that we'd find a significant use of bridges over streams and rivers by all wildlife. The data however showed us that wildlife used just over one fourth of these bridges and that birds and bats used them equally. Second, we were also surprised to find that over 60 percent of the overpasses (bridges over railroads or roads) were used by birds and/or bats; we didn't expect to see this high of use. Lastly, we found that birds used over 50 percent of the bridges over lakes and that bats used none of them. While we cannot make any hard conclusions about these observations they do help us in refining our sampling effort with the hope that we will be able to shed light on these trends.

Where Are We Going With This Project?

From the above information it's clear that a diverse group of wildlife utilize bridges for different reasons. Our understanding of the types of bridges and surrounding habitats will help us better manage the wildlife of Virginia. There may be situations where we would like to either enhance or discourage bridge use by wildlife. By understanding the structural components of the bridge or surrounding habitat that attract or discourage wildlife we can meet management strategies for individual bridges or groups of bridges over larger areas. In a few cases we have found rare or endangered or threatened wildlife using bridges. The use of this artificial habitat may play a key component in helping to promote some rare species and add to their recovery. As the human population grows our need to understand human-wildlife interactions becomes more important in the use of our natural resources. This project is focused on adding to our knowledge and understanding of Virginia's wildlife and how we can coexist.

 

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