Minnesota Department of Transportation

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Research & Innovation

Species from Feces: A New Tool to Identify Bats in Culverts and on Bridges

Need Statement 705


Minnesota is home to eight species of bat, one of which is federally listed as Endangered (Northern long-eared bat; Myotis septentrionalis; NLEB) and one that is proposed for listing as Endangered (Tri-colored bat; Perimyotis subflavus; TCB). Four of Minnesota’s bat species migrate south during the winter months, while the other four bat species (“cave bats”) overwinter locally in natural caves and man-made structures such as mines, sewers, and buildings. Cave bats in Minnesota and elsewhere are threatened by a fungal disease known as white-nose syndrome (WNS). WNS has been the primary driver in cave bat population declines of over 90% in North America. Following these steep declines, the U.S. Fish and Wildlife Service (USFWS) listed the NLEB as threatened under the federal Endangered Species Act in 2015 and in early 2023 reclassified the NLEB as endangered. In addition, USFWS proposed listing the TCB as endangered, which is anticipated to go into effect in winter 2023. It is possible that additional Minnesota bat species may be listed as threatened or endangered by USFWS in the future.
During Minnesota’s summer months, Minnesota’s bat species may utilize bridges, culverts, and buildings as roosting habitat. These structures may be used to harbor large maternity colonies where they give birth and raise their young, or as occasional rest areas used by singletons or small groups of bats. When bats infrequently use the culverts and bridges, their feces – guano – are typically the only evidence present during an inspection or survey. Absent having bats physically present, bridge inspectors and MnDOT biologists are unable to determine which species are utilizing a particular culvert or bridge. As a result, USFWS regulations require transportation agencies to assume the bat use is attributable to one or more of the endangered species (i.e., transportation agencies are expected to err on the side of assuming NLEBs and/or TCBs are using the culverts or bridges). Assuming the presence of endangered bats can complicate regulatory compliance and may delay environmental review processes and add costly mitigation obligations.


Collecting bat feces in the form of guano for DNA analyses may be a way to avoid unnecessary project delays and project mitigation, as the DNA analyses can confirm which species are utilizing the structures even when bats are not physically present during an inspection. This concept has been proven and utilized in Minnesota and other states and has succeeded in determining structure use by protected bat species, including NLEB. This proposed study will further refine techniques and sampling protocols and develop a standard methodology for use on transportation structures in Minnesota.

Strategic priorities

  • Innovation & Future Needs:
    The purpose of this proposed project is to develop and validate protocols for sampling bat guano on culverts and bridges to improve environmental compliance and reduce excess regulatory burden for transportation agencies in Minnesota. Endangered bat species are inherently scarce among the landscape – suggesting that the DNA analyses would confirm that endangered bat species are not utilizing substantial numbers of transportation structures (e.g., bridges or culverts), which in turn would streamline Endangered Species Act project reviews for projects impacting these structures. Anticipated benefits to MnDOT may include:
    • Reduction in regulatory constraints
    • Fewer restrictions on bridge and culvert work timing
    • Streamline and expedite Endangered Species Act (Section 7) consultation with USFWS
    • Avoid lengthy Formal Consultation with USFWS
    • Enhancing MnDOT project delivery
  • Climate Change & Environment:
    As a result of this project, MnDOT will contribute to knowledge gaps related to the use of transportation structures by endangered bat species. This project will result in improved conservation outcomes for bats in Minnesota, bats that provide a wide array of ecosystem services (e.g., pest insect control).

Expected outcomes

  • New or improved manual, handbook, guidelines, or training
  • New or improved policy, rules, or regulations
  • New or improved business practices, procedure, or process
  • New or improved tool or equipment

Expected benefits

  • Construction Savings, achieved by a reduction in time
    Identifying bridges that do not have use by endangered species is expected to reduce regulatory burdens associated with project and maintenance activities involving bridges and culverts and may eliminate or reduce the need for of seasonal timing restrictions related to bridge and culvert projects. Verification that endangered bats are not occupying transportation structures will reduce costs associated with project-specific minimization and mitigation associated with bat impacts.  
  • Decrease Engineering/Administrative Costs 
    Avoiding formal consultation with USFWS will reduce administrative costs associated with project development and delivery.
  • Environmental Aspects
    Improving the understanding of bat use of transportation structures will improve conservation outcomes for bats by allowing transportation agencies to focus impact avoidance and other conservation efforts where they are most needed.
  • Operations and Maintenance Savings, Reduced time
    Identifying bridges that do not have use by endangered bat species may result in more permissive maintenance activities and timelines.
  • Reduce Risk
    Early identification of bat species using transportation structures can reduce regulatory risk. Federal undertakings require U.S. Fish and Wildlife Service authorization to proceed, and delays in identifying endangered species presence can jeopardize project schedules and funding.
  • New method of using technology
    Use of DNA analysis to determine species use is novel.

Supplemental Notes

Does the project scope include a need to detect the fungus that causes White Nose Syndrome (WNS) in bats?  No. Focus is to develop and validate protocols associated with the determining the presence/absence of endangered bats. In Minnesota, bats typically use bridges and culverts during the summer months, and not for hibernation.

Is the focus on only the two endangered species, the northern long-eared bat and tricolored bat? The goal of the project is to be able to determine whether these two rare bats use or do not use a particular structure. A protocol should include some measure of confidence, e.g., 90% or 95% confidence of absence based on validated sampling protocol.

What bats are observed on bridges and culverts in Minnesota? Prior to the onset of WNS disease in Minnesota, bats observed on transportation structures include, in order of prevalence, little brown bats (Myotis lucifugus); big brown bats (Eptesicus fuscus); tricolored bat (Perimyotis subflavus); and northern long-eared bats (Myotis septentrionalis). A single instance of a silver-haired bat (Lasionycteris noctivagans) in a highway tunnel has also been documented in Minnesota. Following WNS associated declines, big brown bats have become the most common bat observed on transportation structures followed by little brown bats. Northern long-eared bats and tricolored bats have not been observed on transportation structures since 2018 and 2019 respectively.

What is the current bat-bridge/culvert process look like? Minnesota DOT has incorporated a new inspection element (Element #900) during its routine inspections. If inspectors identify a bridge or culvert has having bats, MnDOT wildlife biologists will follow up with an more detailed inspection to determine which species are present. Currently, biologists focus on visual inspections of physical bats to determine species. However, evidence of bat use (e.g., guano) is present more often than bats themselves. In these situations, biologists may use eDNA methods to determine which species have used the structure. However, sampling methods have not been validated in a way that can be used to confidently determine species absence.

Are bats typically found in expansion joints in Minnesota? Most day roosts occur in expansion joints. However, tricolored bats may day-roost elsewhere (e.g., on vertical surfaces on beams). In addition, all bats may night-roost outside of expansion joints on vertical surfaces.

Is MnDOT interested in the use of drones for bat inspections or guano sampling? The project’s main goal is to validate a sampling method that can confidentially determine presence/absence of bats on a structure. MnDOT does use drones for bridge inspections, but that is not the focus of this project. Drones may have a role to play in collecting guano samples from difficult to reach locations and/or when other equipment (e.g., bucket trucks) are unavailable.

Does MnDOT need the ability to have DNA results in the field? No. Investigations are typically completed 2-3 years ahead of the start of a project, so waiting for laboratory results is acceptable.

Technical advisory panel