Birds and Bridges — A Look At the Upper Mississippi

Nancy Duncan, Natural Resource Program Manager, Mississippi National River and Recreation Area The National Park Service has teamed up with Fordham University to conduct the first study of its kind exploring the potential risks that bridges pose to migrating birds along the Mississippi River.

Many people are surprised to learn that most birds migrate at night. For unknown reasons, night-migrating birds are attracted to isolated and/or bright sources of artificial light. Once birds enter such light sources, they can become confused and will often circle in the light. Unfortunately, this behavior can result in exhaustion and collisions with human structures – often leading to death. Collisions with buildings are the second leading cause of bird mortality in the US – killing between 100 million and 1 billion birds every year.

While the impacts of buildings, communications towers, and at-sea oil rigs on night-migrating birds are well documented, we know little about the impact of bridges. The intention of this study is to address this knowledge gap. To explore the issue, we used small-scale avian radar along with acoustic recorders to track birds at several bridges during spring migration. The study, which lasted about a month, focused on three bridges in the Mississippi National River and Recreation Area. The bridges were chosen after careful consideration of multiple factors such as height, lighting, and orientation.

This study used vertical and horizontal avian radar to determine how many birds were flying near the bridges and what height and direction they were flying. The radar also examined whether the birds were being drawn to the bridges, if they changed the direction of their flight, and whether their trajectory was actually in a collision course with the bridge. Acoustic recorders monitored the nocturnal flight calls of birds, which will help researchers identify the species of birds that are flying over the bridges.

If the study reveals that bridges in fact have an impact on migrating birds, hopefully it will inspire action to do more for these birds on their migratory journey.