So, now that you’ve decided which detector to purchase . . .
. . . how do you use it?
All of the different detectors can be used to determine the presence or non-detection of bats in an area. To do this, it is important to first recognize the difference between typical bat calls as heard thru a bat detector and sources of other ultrasonic noise (e.g., insect calls (katydids, crickets, cicadas), motor noise (from cars, generators, etc.), movements of people (especially people wearing synthetic fabrics that rub against one another), and the ultrasonic pulses made by some auto-focus/auto-exposure cameras).
Before deploying an acoustic survey, it helps to familiarize users with how different species will sound through a detector and to obtain reference calls from known species to compare to unknowns that may be recorded in new areas.
To determine behavior it is often important to have a detector you can listen to in real time, in order to place the call with the bat’s action. In this case, heterodyne and frequency division detectors can be useful. It is easy to determine if a bat is simply flying thru an area or if it is actively foraging in an area by listening for the feeding buzzes (remarkably speeded up calls which all run together like a zip instead of individual tonal chirps).
All detectors can be used to monitor activity patterns when placed and monitored in the same locations for a standard sampling period throughout the year. Results from studies such as these may give clues as to when bats move thru and/or are resident in an area.
Relative abundance can be investigated by conducting walking or driving transects through different habitats. Presumably, the same bat will not be following a moving detector so numbers and types of calls detected can yield important information about how bats differentially use various habitats.
It is not possible to completely quantify exact number of bats in each habitat, only that bats are x-times more common in one habitat when compared to another. It is important to fully document weather and moonlight conditions during monitoring periods because both can affect bat activity. If possible, estimates of insect biomass should also be taken, as this can affect bat activity as well.
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Summer Acoustic Transects have definite plusses: Capture (and therefore WNS decon) is not necessary.Acoustic inventories have been proven to be very effective survey techniques, especially in Europe.No experience is needed to conduct transects and volunteer labor can be used.Using FS/DR detectors, multiple species can be sampled simultaneously.
Each methods has specific pros and cons and best uses. Summer PassiveAcoustic Point Counts have definite plusses: Capture (and therefore WNS decon) is not necessary. Acoustic inventories have been proven to be very effective survey techniques, especially in Europe. No experience is needed to conduct transects and volunteer labor can be used. Using FS/DR detectors, multiple species can be sampled simultaneously.
There is much discussion about using Acoustic Monitoring to assess post-WNS bat communities on the summer landscape. There are two main ways this can be accomplished:
1. Through Summer Mobile Transects and
2. Summer stationary passive point monitoring.
Each method has pros and cons and suggestions for appropriate use
Increasingly, bat detectors are being used to monitor the affects of WNS as it expands across the Northeast, westward and southward. One of the best indicators of an infected hibernacula is unusual mid-winter bat activity. Deploying detectors passively outside these sites is an easy way to measure the potential for WNS at new sites.
During the winter of 2009 / 2010, BCI assisted KDF&WR with deploying AnaBats at 8 sites of special concern. Units were housed in weather proof boxes and powered by 12-volt batteries that were re-charged by 10W solar panels. Boxes were attached to trees within 30m of the entrances and paired with Hobo temperature data loggers to record ambient temperatures hourly.
Knowledge about the number of bat passes detected does not allow for an estimate of the number of bats present in a study area because there is not a one to one relationship between the number of bat passes and the number of bats responsible for those passes. That is, it is not possible to discriminate between several bat passes made by a single bat flying repeatedly through the study area versus several bats each making a single pass. Therefore, bat passes do not allow a direct estimate of population densities. However, the technique does allow a relative measure of bat activity in an area and allows for comparisons between areas or over time to be made.
Deploying detectors in different habitats on the same nights will provide information about relative abundance. Deploying detectors in the same habitat all season will provide information on how bat diversity and abundance varies temporally. Linking this type of data with insect trapping and microclimate will shed additional light on factors influencing bat activity. By monitoring changes in relative bat abundance over time, temporal changes in activity patterns can be assessed and population trends identified.
Estimates of bat abundance (activity counts) using bat detectors are indices of population size, not absolute measures of abundance or density. To monitor changes in relative abundance, it does not matter if it is impossible to determine whether a count of five bat passes corresponds to five different animals or to one animal passing five times. Provided the average number of passes per bat does not change over time, the number of passes can be taken as an index of the number of bats.
If the number of passes declines by 50% over a 5-year period, and other factors remain unchanged, then we estimate the number of bats has halved.
Deploying detectors spring and fall can provide key information about the timing of migration, something that is still poorly understood in bats. Significant increases in bat passes can indicate major movements through a landscape.
Identifying bats to species, or guilds, and associating activity with meteorological data during these times can make this information even more valuable.
Comparing routine commuting calls with foraging “feeding buzzes” can identify areas of travel vs. of rich food resources and provide key information about critical habitats.
Comparing areas where feeding buzzes are recorded with insect trapping can provide even more insight into bat diets.
As bat populations decline throughout the world, understanding habitat use is becoming increasingly important. While data from bat detectors cannot be used to determine the number of individuals present in the area, there are several methods for defining the amount of bat use in an area.
The most common measure of bat activity is the number of files or passes. Bat activity should be used as a relative term for assessing bat use. In some areas, 30 passes may represent high activity while in others 100 may represent a low activity level. No matter the technique used, the researcher must explicitly define the means by which bat activity is measured and discuss the implications of this definition on the results.
So, how many call-pulses in a recording equal a bat pass? A lot of researchers would say “one” but whatever metric you use, make sure it stays constant throughout the life of the monitoring project. The same is true when you start trying to identify bat passes to species and make classification decisions. Be sure that whatever call parameters you apply to a collection of recordings remains the same over time.
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