Bridge maintenance and construction projects in Minnesota can threaten endangered bats that roost beneath the structures. However, identifying which species are habitating under a bridge can be difficult since they may not be visible during routine inspections. To improve environmental compliance and streamline regulatory processes for transportation agencies statewide, University of Minnesota researchers are testing a new method to identify bat species: analyzing their feces, or guano.
In a project led by Associate Professor Ron Moen of the U of M Duluth and sponsored by the Minnesota Department of Transportation (MnDOT), a team aims to develop a guano sampling protocol for determining what bat species are present under bridges and which structures are most likely to be used by bats. In a recent webinar hosted by MnDOT and the Minnesota Local Road Research Board, Ryan Foley with the environmental assessment unit of MnDOT’s Environmental Stewardship Office discussed the connection between bats and transportation infrastructure as well as what the research has found to date.
Bats provide important benefits, but their populations are declining worldwide because of human activity and white-nose syndrome—a deadly fungal disease that’s killed millions of bats since its discovery in 2006, Foley noted. In Minnesota, three species have been hit especially hard. Under the federal Endangered Species Act, the northern long-eared bat is listed as endangered, the tri-colored bat is proposed for listing, and the little brown bat is currently under review. Protecting endangered bats requires, in part, conserving their habitat.
Foley explained that it’s common for bats to roost in the nooks and crannies of bridges, feeding at night or establishing maternity colonies. Some even return from hibernation to the same maternity habitat each year to raise their young. For bridge projects, regulations require transportation agencies to assume endangered bats are present if guano is found—complicating regulatory compliance and increasing construction and maintenance costs.
Researchers have collected and are analyzing guano from 92 bridges in Minnesota. They’ve also deployed acoustic data collection at 54 bridges, which can be used to cross-reference results from guano pellet collection. To date, about half of the bridges had pellets from little brown bats, and about half had pellets from big brown bats. Eight species have been identified using acoustic data, including the northern long-eared bat and the tri-colored bat.
Almost all bridges on which bat guano has been found are concrete structures, which Foley explained could be due to thermal inertia—bats using bridges at night to stay warm—with over 90 percent spanning waterways. Longer, taller bridges have a higher probability of bat use, likely because they offer more potential roosting sites. Initial results also show some distinctions in how different species use different bridges, with some evidence suggesting that certain species might selectively use bridges rather than nearby forests.
As this research continues, it offers a promising path to more cost-effective protection of endangered bats during bridge projects. The team plans to deliver its final guano sampling protocol in 2026.
—Krysta Rzeszutek, CTS digital editor
