At a glance: NABat Stationary Acoustic Surveys
The goal of stationary acoustic surveys is to capture the full scope of bat species diversity within a GRTS cell. This is accomplished through presence-absence sampling, meaning a single positive identification of each species is all that is required at each sampling location. Therefore, the priority of stationary acoustic sampling should be to acquire high-quality recordings that can be confidently identified, rather than focusing on recording a high volume of recordings.
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1. Select GRTS cells to survey. Acquire landowner permission, if applicable. Find a sample property access request letter here.
2. Select sites to monitor. Each 10 x 10 km GRTS cell is comprised of four 5 x 5 km quadrants; users should place no more than 1 acoustic detector per quadrant and survey at least two quadrants per grid cell. Homogenous landscapes may only require two quadrants to capture the species diversity within a cell. If landscapes are heterogenous or unfamiliar to the user, place detectors in up to four quadrants to ensure the major habitat types are represented in sampling. Site selection is a multi-year commitment that should be diligently assessed by knowledgeable biologists using on-the-ground reconnaissance.
Note: Aim for call diversity, not quantity. It may be necessary to place one or two detectors in habitats that may be used only by one or two species if those habitats are the most likely area to host those species
3. Prepare survey equipment. Most bat detectors are compatible with stationary acoustic monitoring; however, time-expansion detectors should not be used. To standardize recordings, recommended settings involve a two-second trigger window and a maximum file length of 15 seconds.
4. Conduct surveys. Monitoring should occur during the summer active period prior to the young becoming volant. This target period may vary with location or species. When possible, surveys should be conducted when weather conditions are optimal for bat activity. Each survey point should be sampled at least once per year, for a minimum of four consecutive nights. Surveys should last the entire night, from 15 minutes before sunset until 15 minutes after sunrise.
5. Review full NABat Stationary Acoustic Survey protocols in A plan for the North American Bat Monitoring Program (Chapter 4).
At a glance: NABat Mobile Acoustic Surveys
The goal of mobile acoustic surveys is to estimate the relative abundance of bat species within a GRTS cell.
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The latest Mobile Acoustic Transect Surveys Standard Operating Procedures (SOP) are available for download at the USGS publications warehouse here.
Link directly to the Mobile Transect SOP for Conducting Mobile Transect Surveys
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1. Select GRTS cells to survey. Acquire landowner permission, if applicable. Find a sample property access request letter here.
2. Delineate a survey route. Routes should cover 25-48 km (~15-30 mi) on roads where a constant speed of 32 kph (20 mph) is possible with little to no stopping. Plan a route that covers the cell’s predominant habitat types, but avoid roads with heavy traffic (for safety) and dense forested corridors (for call quality).
To ensure each bat recording is a unique encounter, avoid routes that loop back near previously surveyed areas. If unavoidable, aim for all route sections to be separated by > 100 m lateral distance.
Note: Mobile transect routes should be targeted toward a single GRTS cell, but may have sections or an endpoint that extend beyond the GRTS cell boundary into an adjacent cell(s).
3. Prepare survey equipment. Acceptable bat detectors include full-spectrum (preferred), zero-cross, and frequency division. Zero-cross detectors that auto-level the noise floor should not be used, nor should time-expansion detectors. Directional microphones are preferred and should be affixed centrally on the vehicle rooftop, preferably aiming their cone of detection upwards.
All recordings should be georeferenced. Most detector types are capable of writing GPS metadata, either through internal GPS mechanisms or through the attachment of accessory GPS units.
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Note: Because mobile transects should only be conducted during fair weather, microphone weatherproofing is not necessary and should not be used (for call quality).
4. Conduct surveys. Users should perform mobile surveys twice per year during the maternity season — ideally, the second survey will be conducted within a week of the first. The window of appropriate survey dates may vary based on the phenology of regional bat communities. To maintain consistency in subsequent years, future surveys should be conducted within one-two weeks of the original survey date.
Begin mobile surveys 45 minutes after sunset. If the survey vehicle must stop for any reason, the detector should be paused until the survey resumes.
Note: Ensure detectors are functional for the duration of the survey by generating an ultrasonic test noise (e.g., rubbing fingers together, jangling keys) at the microphone immediately after beginning and prior to ending the survey.
5. Review full NABat Mobile Acoustic Survey protocols in A plan for the North American Bat Monitoring Program (Chapter 5).
At a glance: Colony Count Surveys
The goal of colony count surveys is to estimate population abundance in gregarious species or to record species whose call characteristics are not suited to acoustic monitoring. Bat colony sites can be sensitive locations, NABat offers multiple levels of security for when site secrecy is a concern.
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Disclaimer: NABat recognizes that many ongoing colony monitoring programs consist of years or decades of effort. NABat guidelines encourage survey effort consistency to create comparable datasets; however, existing colony monitoring programs should not significantly alter their current protocols unless their data reliability would be significantly improved. Consistent long-term monitoring is more important than full integration of colony count efforts into the NABat framework.
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Internal Colony Counts
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1. Assess the need for an internal colony count. Entering a roost may negatively impact bats and should be avoided when possible. An internal colony count may be appropriate if external counts cannot be conducted or cannot reliably identify a roost’s species, or if data needs require winter-specific population estimates to assess threats associated with hibernacula (e.g., WNS). To ensure reproductive success is not compromised in maternity colonies, internal surveys of summer roosts are not recommended unless absolutely necessary.
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2. Establish a survey sample design. If a regular sampling interval can be maintained (≤ 3 years between surveys), it may be most appropriate to monitor all colonies in a region. Alternatively, GRTS cell selection protocol can be used to prioritize monitoring when the number of colonies exceeds monitoring resources in a region.
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3. Conduct surveys. Whenever possible, winter surveys should be conducted between late January and early March. To minimize disturbance, conduct surveys no more than once per season — some species-specific guidance may recommend surveying once every other season. If internal roost counts must be conducted in the summer, surveys should occur during the final two weeks of pregnancy. Entering roosts with non-volant pups incurs a high risk of pup mortality.
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Multiple-observer surveys are recommended to minimize bias in abundance estimates. However, it is critical that data collected in this manner be independent (i.e., no communication between observers). Surveyors must also consider that duplicated effort has the potential to increase disturbance within a roost and should make every effort to minimize disturbance.
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Digital photography is the preferred method for internal colony counts as it minimizes time spent in a roost and creates a permanent record of the survey. Performing a visual survey in tandem with digital photography generally results in a reliable colony count. Cameras must be capable of producing clear images where individual bats are easily distinguished. Cluster abundance may be estimated by extrapolating bat density estimates over the cluster area; in such cases, packing density should be estimated for each cluster and for multiple subsets of large clusters.
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Note: It is crucial that large roosts be subdivided into named sections and bat counts be attributed to their respective section. Spatial inconsistency between surveys often renders incomparable data; however, comparisons may still be drawn at the roost section level when such data exists.
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External Colony Counts
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1. Establish a survey sample design. If it is practical to monitor all known colonies in a region each season, it may be most appropriate to continue monitoring all colonies. Alternatively, GRTS cell selection protocol can be used to prioritize monitoring when colony prevalence exceeds monitoring resources in a region.
2. Assess whether an external colony count will result in accurate estimates at a roost. Emergence counts are appropriate when all roost exits are known and can be monitored simultaneously. Conditions must allow individual emerging bats to be identified; additionally, species compositions and relative abundances of the roost must be known beforehand.
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3. Conduct surveys. Emergence counts are most productive during late pregnancy or early lactation, prior to young becoming volant. Performing two-three emergence counts during this period typically produces reliable data. In most scenarios, it is appropriate to begin monitoring a roost 30 minutes before sunset and continue at least 10 minutes after the last bat emerges or until it is too dark to see. However, larger colonies may emerge in pulses with periods > 10 minutes between emerging bats.
Capture Records
NABat encourages submissions of capture data; user capture data may facilitate the interpretation of acoustic and colony count surveys to verify species presence/absence in an area. However, NABat does not specify guidance protocols regarding the capture or handling of bats. Capture surveys should only be performed under direct supervision of a permitted biologist.