Understanding the spatial and temporal distribution of bats has
recently become an important priority for land managers and
biologists throughout the world as energy use switches from fossil
fuel to renewable energy sources such as wind power, which can
increase mortality rates. A
recent Government Accountability Office (GAO) report (2005)
indicated that wind power has dramatically grown in the
United States
, with further growth expected.
The GAO recommends that FWS provide state and local
regulatory agencies with information on the potential wildlife
impacts from wind power. However,
such information is lacking because significant gaps in the
scientific literature make it difficult for scientists to draw
inferences about wind power’s impact on wildlife in general.
Currently,
three different technologies are used for bat acoustic monitoring:
heterodyning, frequency division, and time expansion.
Acoustical devices accomplish two major tasks:
they bring an ultrasonic signal into the human audible range, and they
record signal so that they can be analyzed in research investigations.
Heterodyning uses a narrowband technique, i.e., devices are tuned to a
narrow frequency range. The
difference between the center frequency of the tuned range and the incoming
signal is audible, and this audible signal can be heard in real-time.
Sensitivity is high because only a narrow band of frequencies is being
transformed, resulting in a narrow band of noise.
However, bat calls outside of the tuned range are not detected at all.
In contrast to heterodyning, frequency division monitors the entire
ultrasonic range. Incoming calls
are divided by a constant factor, shifting the frequencies down into the human
audible range and allowing the signal to be heard in real-time.
As with heterodyning systems, frequency division systems record only
the altered signals. Time
expansion units record the ultrasonic call directly and then play it back at a
slower speed to decrease the frequency. Calls
cannot be heard in real-time; however, the original signal is retained in its
entirety.
In light of recent wind-energy issues, understanding the temporal and
spatial distribution of bats across large spatial extents has taken on new
urgency. In this study, we
conducted field trials to assess and compare the performance of three commonly
used acoustical units for monitoring bat distribution patterns:
Pettersson D1000x (Pettersson Elecktronik AB,
Uppsala
,
Sweden
), Anabat II (Titley Electronics, Inc,
Ballina
,
Australia
), and AR125 (Binary Acoustic Technology, Inc,
Arizona
,
USA
) under different conditions. Field
trials were conducted in which high- and low-frequency bat calls were
transmitted at varying distances and angles from the three types of bat
receivers. Trials were conducted
in open versus vegetated habitats, and receivers were assessed with and
without sound-reflecting surfaces. Our
objective was to compare detection rates (detect versus not detect) among
different units as a function of (1) the distance to an emitted bat call (25
– 200 feet), (2) the orientation of the units with respect to the bat call
(0 to 90 degrees), (3) the type of bat call (low versus high frequency), (4)
the presence or absence of a sound-reflecting surface used to mimic
commonly-used rain-guards, and (5) the presence or absence of foliage.
