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Well, of course, it depends. In areas of low bat diversity or where bats have very distinctive echolocation calls . . . either in minimum frequency or duration . . . even a simple HET detector will work for ID. Responses at different dialed frequencies and their tonal components can give plenty of information to assist with species ID. The realtime nature of the recording also provides additional identification cues if the bat can be observed while the recording is being made.
FD/ZC detectors are very effective because they do not need to be manually dialed and are responsive to the entire frequency/time components of a bat call. They are especially effective in areas and during recording conditions that ensure the intensity of the incoming bat calls is well above that of the back ground noise.
TE full spectrum detectors (with the important caveats we’ve already discussed about recording relative abundances in areas of high bat activity) and DR full spectrum detectors will record the highest quality calls, therefore capture the most amount of information that can be used to make confident species identifications.
The best detectors to use for recording voucher calls to prepare a call library are those that allow for simultaneous recording whilst viewing the bat in flight . . .
. . . AND those that easily allow the recording to be associated with “text header” information describing the species being recorded and the conditions of the recording.
The best units are the ZC AnaBat and the TE units that allow direct recording to a laptop so text header information can be entered, or recording to a suitable MP3 player where “voice notes” can be entered describing the previous recording (then the crucial “text header” information can be entered later).
Because mobile transects are essential moving passive efforts, the types of detectors that are suitable for them are the same as those suitable for passive recordings.
Using ZC or DR units allows more potential for confident species ID.
Detectors that can be deployed easily in conjunction with GPS units are the most helpful because exact locations for each recording can be documented as well. Software is availble from Myotisoft that syncronizes GPS tracks with your time-stamped bat files.
How DO TE detectors compare to other recorders FOR PASSIVE RECORDING? This table summarizes the a 3.5 hour recording window with variable bat activity, where we simultaneously recorded with a ZC AnaBat, a TE D240x, and a DR D500x.
In theory: With a 17-second delay, the D240x can handle up to four calls per minute, extrapolating this out into 15-minute monitoring intervals, the D240x should be able to collect just over 50 calls. So any time activity is below 4 calls per minute, theoretically D240x will not lose much data. But in practice, how does the D240x fare? After all, bat activity is neither constant, or regular, in fact, bat detectors may return regular bursts of activity.
We looked at this question at a recent workshop in New Mexico where we paired a Zero-crossing (realtime) AnaBat unit with a Time-expansion (quasi-realtime) Pettersson D240x setup, and a Full-spectrum (realtime) Pettersson D500x detector. The goal was: (1) to determine the relative number of calls collected with each system, and (2) to determine at what index of bat activity the D240x began to return fewer calls due to the time-delay of expanding each recording.
What we found during this particular recording session, was that the activity level before the D240x started loosing calls was surprisingly low. It took as few as 5 calls per 15-minute interval before the time-expansion detector could not keep up. When bat call activity was upwards of just 15-20 calls per 15-minute interval, the D250x could process only half as many recordings. So, clearly, if indices of activity are desired, a TE detector is NOT the most suitable model.
Though all detectors can be used passively (if they are hooked up to appropriate recording devices), the most useful are those with on-board storage capabilities.
This includes the AnaBat SD1 and SD2 models. And all the DR units . . . though the AR125 needs to be connected to a netbook, laptop, or FR125 recorder.
The TE full spectrum detectors have been used effectively by many people to record passively. But, the very nature of the TE detector (spending 10-times the time of the bat call expanding (slowing down) the call, means that during times of high bat activity, the detector will be busy expanding and recording and not available to collect incoming calls. As long as absolute values for bat activity are not required, TE detectors can be used, with the understanding that as activity increases, their responsiveness will decrease.
There are trade-offs to active vs. Passive monitoring. Because active monitoring involves the researcher being present to adjust the orientation of the microphone relative to the bat and to manually save detected echolocation calls, active monitoring permits contact to be maintained with the bat, resulting in improved call quality. Active monitoring is most effective with a realtime display bat detector.
If used with a TE detector, new calls may be missed while the current call is being expanded and downloaded to a saved file. Because of this active monitoring with a TE detector is effectively sampling the airspace only 10% of the time. This may seem like a significant impact to the number of calls collected during a monitoring period, but in reality, when the airspace is not saturated with bat calls (as it usually isn’t) it does not really significantly impact studies investigating relative abundance or activity.
Passive monitoring involves the automatic recording of echolocation calls without an observer present, based on predefined deployment criteria. Unfortunately, from night to night, the directionality of the microphone in a fixed (passive) position can exert a strong influence on sound quality and on the quantity of calls recorded. As a bat passes over a stationary microphone, it will be properly oriented towards the receiver for only a fraction of the time it would be if the unit moved with the bat as during an active survey. The result is a much reduced quality of call and some calls may be missed entirely. Passive monitoring, while still permitting species identification, increases the number of calls that cannot be identified. Thus, a tradeoff exits between call quality and the benefits of simultaneous sampling.
The same is true for “active surveys” where visual cues from bats in flight help us to identify the calls they produce. Therefore, some sort of audio or visual output is required. FD/ZC, HET, and some FS detectors can do this in realtime.
TE full spectrum detectors can only be used in quasi-realtime due to the time-expansion factor. Calls can be viewed 10-30 seconds after they have been produced.
The best detectors for Interpretive or Public Education use are those that provide instant audible and/or visible outputs.
These are predominantly HET units like the Pettersson D100 or Pettersson D200
Inexpensive FD units like the Pettersson D230 and Baton can have live outputs onto laptop screens. The AnaBat with an AnaPocket is another FD solution.
Certain TE or FS detectors hooked up to a laptop with analysis software can work too, notably the BAT AR125, the Wildlife Acoustics EM3, and Pettersson D240x..
Before selecting a detector type, think about what use(s) are required. Basically, we can use detectors for:
1. Public Interpretation - Education - or - Citizen Science Programs
2. Active Surveys (where we listen to and/or watch the detector output WHILE observing the bat flight and behavior.
3. Passive Surveys (where we leave detector(s) out on the landscape unattended to record bat passes for a night, a week, a month, a season, or a year or more.
4. Mobile Transects (where we move across a landscape, habitat, region; collecting bat echolocation calls continuously, or at pre-defined stops along a route)
5. Collecting “voucher calls” of known species for a call library
And performing all these actions in attempts to . . .
6. Identify different species of bats based solely on their echolocation calls
The sophisticated detector user, who invests a great deal of time and attention to the equipment and its use in the field, both to characterize calls from known species, and to actively monitor bats to associate call types with observed morphologies and flight types, can use any detector style or brand to return confident species identification decisions. Learning curves with the equipment varies directly with the complexity of the bat fauna being investigated and can be further confounded by challenging recording habitats or situations.
Remember the basic detector types are:
HET = heterodyne (aka, dial a bat)
FD = frequency division (also called ZC = zero crossing)
TE = time expansion full spectrum (slows and lowers calls for analysis and audible output)
FS = full spectrum (records everything all night, but usually can only be used passively)
Choosing the Right
Bat Detector for the Job
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