The study would “determine the effect of bridge size and design on bird strike mortality, particularly with bridges spanning rivers and intersecting migratory flyways,” according to NPS.
This project was the result of several designs that were being considered to replace the old Hastings Bridge, located southeast of Minneapolis, including one with a high arch. The four-lane replacement bridge is “the longest free-standing tiered-arch bridge in North America,” according to Mn/DOT.
However, the NPS Mississippi National River and Recreation Area (MNRRA) expressed concern in a letter that because “the arch bridge will contain cables, there is potential for bird strike impacts.” The letter did acknowledge that because “the issue has not been carefully researched anywhere, the significance of potential bird strike impacts is unknown.”
The NPS then requested “a post-construction research project concerning bird strikes” as mitigation for the project’s potential impact on avian populations. Mn/DOT officials agreed to conduct the study when the arch bridge was chosen as their final design.
However, bridge strikes are not even mentioned in The Sibley Guide to Birds’ list of the top causes of bird mortality. Collisions with windows and attacks by feral cats are the major causes of bird mortality, followed by high-tension wires, pesticides and cars.
MNRRA Superintendent Paul Labevitz says he was pleased that the arch design was chosen for the replacement bridge, saying the first design proposal, was “a giant dream catcher for migratory birds.”
It “had a center spire that was like 270 feet high that had cables that came off it almost like a spider web, and because of the Mississippi River flyway and the fact that this bridge was in the National Park, we were worried about its impact on migratory birds,” he told CNSNews.com.
While there is not a great deal of research on bird /bridge collisions, ornithologists’ opinions vary widely on the usefulness of such studies.
Dr. Robert Zink and Dr. Todd W. Arnold of the University of Minnesota recently co-authored a study which concluded that “collision mortality has no discernible effect on population trends of North American birds.” However, their studies did not specifically include bird/bridge collisions.
When asked about the bridge study, Zink told CNSNews.com, “We should be spending money on trying to worry about serious threats to bird populations. And yes, a few birds collide with bridges, a few birds collide with wind towers, but no one has ever thought that these are significant threats to populations.”
“A lit bridge isn’t going to be an obstacle to birds,” Arnold agreed. “Things like being scared by something else and having poor visibility, you’d need something like that to make a flock of waterfowl fly into a bridge. They wouldn’t do it otherwise.”
However many other ornithologists disagree with Arnold and Zink’s assessment about the threat bridges pose to migrating birds, pointing to the lack of available data on bird/bridge collisions.
Dr. Christine Sheppard, Bird Collisions Campaign Manager at the American Bird Conservancy, believes that the NPS study will add useful data to help prevent bird mortality.
“Lots of water birds fly along rivers and so forth and could be anticipated as collision victims for particular types of bridges in particular places, so what exactly you might expect to collide may vary from structure to structure and place to place,” Sheppard told CNSNews.com.
“I’m glad that people are starting to look at bridges,” she added. “I’m sure that there are ways to make bridges that are safer than other designs once we take a look and actually have some data.”
Labevitz also believes the bridge collision study will be useful because the area is “the most active migration route on the continent.” He added that he hoped that “as bridges are replaced, this issue comes up. Maybe this study will help plug in migratory bird concerns as the design process for bridges.”
The decision to replace the old Hastings Bridge with an arch design also sparked local controversy when Mn/DOT insisted on a supposedly more bird-friendly box girder design for another bridge project in Winona.
“In the case of the Hastings bridge, where the FWS (Fish and Wildlife Service) could have totally removed one avian obstacle by successfully insisting that the old bridge be replaced with a box girder bridge, the FWS approved the arch bridge on the condition that Mn/DOT provide $100,000 for migratory bird conservation programs,” Chris Rogers of the Winona Post wrote.