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Desert Road

Mobile Acoustic Survey Guidance

What are mobile acoustic surveys?

  • A mobile acoustic survey involves mounting a bat detector on a vehicle and driving it along a specific survey route at a particular time of year and night to record bat calls throughout an area. It is essentially an acoustic line transect.

 

  • Mobile acoustic surveys were developed in Sweden in the early 1980s.
     

    • Researchers determined that mobile surveys could “[secure] large amounts of data on abundant species for comparing areas or measuring population trends” (Ahlén 1980).

 

  • “Line transects are used to collect quantitative data about populations and the efficiency varies with species. Especially the car version can give great amounts of data on a few abundant and easily identifiable species but almost nothing on rare species or on species that avoid roads.” (Ahlén and Baagoe 1999).
     

What is the purpose of mobile surveys?

  • Mobile surveys are another tool in our monitoring toolkit, complementing other survey methods like stationary acoustic monitoring, emergence surveys, and colony counts.
     

  • Mobile acoustic monitoring is appropriate for surveying:

    • common/abundant bat species

    • bat species that do not roost in aggregations or roost in widespread, small aggregations that are difficult to count (e.g., hoary bats)

    • regions where colonies are smaller, less conspicuous, or unknown due to the scale of the landscape (e.g., the western part of the continent)
       

  • Mobile surveys are not designed to determine species’ presence/probably absence, especially not for rare or highly specialized species.
     

  • Mobile survey data provide a metric of relative abundance which allows one to monitor population trends over time.
     

  • With mobile surveys, call data are tied to exact locations, which allows one to assess the habitat associations of bat species. Land managers/researchers can conduct this type of analysis immediately.
     

  • By monitoring population trends and evaluating potential habitat associations, managers can gauge the effects of alterations in the local ecology, such as habitat restoration projects (e.g., removal of invasive species, thinning, prairie restoration, dam removal), wildfire, infrastructure development, and disease, on bat species.
     

  • Mobile survey data can be used to examine local, site-specific questions while also contributing valuable data to the continent-wide North American Bat Monitoring Program (NABat) database, which informs a broader understanding of the relative abundance of bat species on a landscape scale.

What is the process?

  • The survey process has many parts, and while some people may engage with each one, others may only be involved with one or two aspects. Therefore, it is important for everyone to understand each component to ensure that we work well together as a team.

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Mobile Survey Process.png

Mobile Acoustic Survey Protocol Summary

Key Elements

  • These aspects of the survey protocol should be adhered to as closely as possible to limit variation, which improves one’s ability to identify trends of interest with higher levels of confidence.
     

  • Survey time of year: Transects should be run at the same time of year. Surveys should occur during the summer maternity season, after female bats have completed any migratory movements and formed maternity colonies, and prior to juveniles becoming volant.
     

  • Survey time of night: We recommend starting a mobile acoustic survey at a time that is optimal for bat emergence in your area (considering latitude, elevation, etc.) and then consistently surveying at that time across surveys and years.
     

  • While there will be some natural variation in bat activity and surveys might vary slightly in whether they capture peak activity, by consistently sampling at the same time of year and night, this variation will be limited.
     

  • Location Data: GPS tracks should be recorded and uploaded with every survey.
     

  • Transect Identification: Each survey route should have a unique name that is consistent throughout time
     

Flexible Elements

  • These aspects of the survey protocol have some flexibility and can be adapted to your particular situation.
     

  • Route location:

    • GRTS cells: While the NABat protocol suggests selecting a survey route where 85% falls within a single GRTS cell, this is often not possible in areas with few road networks. There is flexibility in this part of the protocol, and it is fine for a survey route to cross into multiple GRTS cells. If at least 500 meters are sampled in a GRTS cell, data from that part of the survey will be used in the analysis of that cell. Think first of the best survey route given the constraints of road availability, and then about meeting the GRTS cell recommendation.
       

    • Switchbacks: The NABat guidance recommends avoiding switchbacks in order to limit the potential for double counting the same individual bat. However, in some cases, it is not possible to avoid areas with switchbacks. Fortunately, the analytical models are robust to switchbacks (even multiple switchbacks for a given route), assuming that, on average the same bat is not counted three times.
       

  • Route length: While the NABat guidance suggests surveying 15 - 31 miles, if the survey route is a minimum of 6.5 miles long, the data can still be used in statistical analyses. Shorter routes are not recommended because they limit the opportunity for detections.
     

  • Survey speed: While the NABat protocol recommends traveling consistently at 20 mph, the analytical models are robust to slower speeds, and it is fine to travel as slow as 10 mph. It is also okay for the survey speed to vary across a transect​. This variation will likely be consistent each time a transect is run. Remember to put your safety first.

What questions can be answered with mobile acoustic survey data?

Habitat Analyses

  • It is possible to use mobile acoustic survey data to analyze the associations of bat species with different types of habitats. This can help inform local management efforts, such as evaluating population trend responses to restoration programs.
     

  • Because a time series is not needed for analysis, habitat analysis can be run immediately.
     

  • When conducting mobile surveys, it is important to concurrently collect accurate GPS tracks. This allows every bat call to be tied to a specific location, which can then be compared with surrounding habitat characteristics.
     

  • A simple habitat analysis compares the distance from a bat call to the nearest patches of different habitat types with those distances for a random set of points taken along the survey route. By comparing these distances, one can determine if a bat species is associated with particular habitat types.
     

Population Trend Analysis 

  • With the mobile survey methodology, one can loosely assume that each call sequence represents a single bat, which provides a metric of relative abundance.

    • This assumption is not necessary, however, as long as there is a consistent relationship between the number of call sequences recorded and the absolute abundance among sampling years, which allows for some flexibility in the data collection protocol.
       

  • With this metric of relative abundance, population trends can be assessed.

Why start now?

  • With mobile acoustic surveys, one typically needs about 5-10 years of data to detect a population trend. 

    • Analysis of European transect data has revealed that a ~1% annual decline of common species could be detected within 5-8 years given modest sampling effort (Roche et al. 2011, Jones et al. 2013). Rare species (~ 1.8 passes per transect) were also able to be monitored, however it is projected to take 10-15 years to detect a 2.5% annual decline (Jones et al. 2013). Catastrophic declines, such as those observed due to white-nose syndrome, only take a few years to observe (Whitby et al 2021).​
       

  • In 2024, Pseudogymnoascus destructans, the fungal pathogen that causes white-nose syndrome, was detected in five counties in California. Now (2025) is an ecologically opportune moment to collect some baseline data on the relative abundance of those bat species that are known to be affected by the fungus before the disease begins to present.
    https://whitenosesyndrome.org/where-is-wns 
     

  • Wind energy infrastructure is projected to grow substantially over the coming decades. Given the threat this poses to those bat species that make long-distance movements, including many of the tree-dwelling bat species, it is critical that, as a community, we begin to monitor population trends.
    https://www.energy.gov/map-projected-growth-wind-industry-now-until-2050
     

  • Data about the relative abundance of bat species will provide land managers and policymakers with science-based information to make and adapt management decisions given complex and changing environmental conditions. 

Frequently Asked Questions

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