Bat Echolocation Researc h - Bat Conservation International

Figure 9: The best listening frequency often coincides with the shallower
part, or the narrowband (QCF) part of a pulse. An even distribution of
energy, making for no best listening frequency, is also diagnostic.
Harmonics
Harmonics occur as a component of the bat’s signal or
even as artifacts of signal processing in the detector or
analysis software. Harmonics can be analyzed using a
time-expansion system and can be heard in the field with
heterodyne detectors. Use of harmonics for species identification,
however, requires more research. In species
where the discriminating narrowband QCF frequencies
are relatively close (e.g., Eptesicus serotinus versus E. nilssonii,
or Pipistrellus pipistrellus, P. nathusii, and P. kuhlii), an analysis
of harmonics, where the ‘gap’ between harmonics is doubled
or tripled, can enhance discrimination (Fig. 10).
Frequency Range
Bat signals are often described with respect to the
range of frequencies in a call, which can be expressed as
bandwidth. Thus, a call sweeping from 80 to 40 kHz has
a bandwidth of 40 kHz. Because atmospheric attenuation
is stronger at higher frequencies (e.g., Griffin 1971;
Lawrence and Simmons 1982), the distance between bat
and microphone influences the presence of high frequencies
in the received signal, resulting in increased
variation in, and often underestimation of, maximum frequency.
The directionality of the microphone and the
‘beam of sound’ produced by the bat are also frequencydependent.
The position and angle of flight direction of
the bat relative to the plane of the microphone membrane
therefore influences the presence of high frequencies
in the received signal. As a result, maximum frequency
is a difficult parameter to measure in the field or
in the analysis of recordings of free-flying bats. The lowest
frequency is less affected by these phenomena. In
narrowband pulses, lowest frequency almost always
coincides with the best listening frequency in the QCF
part of the call (Fig. 6).
Shape or Curvature
The pattern of change in the FM rate or the presence
of real CF, such as in a horseshoe bat’s pulse, gives pulses
a specific character or shape. Species exhibit a range
of possible shapes (Fig. 11) and individuals show flexibility
in relation to the hunting environment and behavior.
FM pulses can be almost linear (e.g., M. bechsteinii, M.
nattereri, M. myotis), curvilinear (e.g., Plecotus spp., M.
mystacinus/brandtii), or bilinear, with a steep, even portion
followed by a shallower part, terminating in a further
rapid sweep (e.g., M. daubentoni, M. mystacinus/brandtii in
open habitat, M. myotis in open habitat). Curvilinear calls
may have a relatively long narrowband QCF ending
(e.g. Pipistrellus sp., Nyctalus sp., Vespertilio murinus, Miniopterus
schreibersii). Bilinear calls may appear as steeper
FM, with a clear angle to the narrowband ending of the
pulse (Eptesicus sp.). In relatively open habitat situations
or while commuting, the shallow part in the middle of
the pulse of, e.g., M. dasycneme or M. myotis/blythii may
become very narrowband (QCF). In open situations, the
narrowband pulse of, e.g., N. noctula may lose its steeper
FM beginning altogether, while the pulse of a species
like M. dasycneme loses the steeper FM parts at the beginning
and end of the pulse. Distinct patterns include the
hook-shaped QCF-FM calls of the barbastelle (Barbastella
barbastellus), beginning with a shallow part and ending
in steeper FM, or the FM-CF-FM calls of the Rhinolopidae
(Fig. 11 – e.g., Ahlén 1981; Kalko and Schnitzler
1993; Limpens and Roschen 1995; Limpens et al. 1997;
O’Farrell et al. 1999; Tupinier 1996; Weid and von Helversen
1987).
Figure 11: Examples of pulse shape.
Figure 10: Harmonics
can help differentiate
between similar bestlistening
frequencies.
52 Bat Echolocation Research: tools, techniques & analysis