Bat Echolocation Researc h - Bat Conservation International

Figure 5: Call sequence showing the frequency shift between two individuals of the same species (Pteronotus davyi) foraging in the same space.
variation for key call parameters such as the maximum,
minimum, and characteristic frequencies (FMIN, FMAX,
FC), as well as pulse duration (DUR).
Without a large series of recordings for even the most
common species, I would not have observed the frequency
shift in calls of individuals. This was noted by
watching calls on the computer screen in real time with
the Anabat system while observing the bats with a spotlight.
Frequently an individual shifts the characteristic
frequency up or down when another conspecific arrives
in the same area. This shift generally ranges from 2-3
kHz. An example of this frequency shift between two
individuals of the same species (P. davyi) foraging in the
same space can be seen in Fig. 5. Observers unaware of
such frequency shifts when conspecifics use the same
space could overestimate the variation in the basic
search-phase calls of an individual or species.
To illustrate this point, I extracted data for call
parameters from the recordings (Fig. 5). If only 15 pulses
were recorded at the beginning of the call sequence,
the range in frequencies would be FMAX=70.8 kHz,
FMIN=58.3 kHz, and FC=67.6 kHz. If only the 11 pulses
at the end of the call sequence were recorded, these
parameters would be FMAX=68.2 kHz, FMIN=58.0 kHz,
and FC=61.6 kHz. If this sequence were recorded using
time expansion, which cannot display bat calls in real
time (Pettersson, pers. comm.), researchers may erroneously
assume that they were seeing variation in the
search-phase calls of one individual.
Like any new endeavor, there must be a starting
point. In the beginning, every call sequence recorded is
by definition, an “unknown species” acoustically. It was
clear there were many different types of calls based on
shapes and frequency. The first task was to match faces
with voices. This meant bats had to be captured for verification
of species identification. Captured individuals
were flown under controlled conditions, either in enclosures
or in the field, to match recordings to positively
identified animals. Chemiluminescent light tags were
often used to allow released bats to be followed with the
detector.
At present, 28 of the 32 species of the non-phyllostomid
bats (7 families) in Belize can be positively
Section 2: Acoustic Inventories
identified acoustically (Appendix 1). However, many
other species that do not occur in Belize can also be
identified acoustically. I have recorded and confirmed by
capture dozens of common species that occur in Belize,
Mexico, Honduras, Guatemala, El Salvador, Costa Rica,
Venezuela, and Bolivia. I have not found geographic
variation in the calls of these species to the extent that
hinders identification after careful comparison to the
Neotropical bat call libraries that are based on verified
species calls.
In the Neotropics, it can be both easy and difficult to
identify bats using echolocation calls. It has been easy
because with the multitude of vastly different families
and genera, call sequences often separate readily. It has
also been difficult because we do not fully understand
the range of species distributions or, in many cases,
species limits in the Neotropics.
For example, there has been an increase in the
number of recognized species since 1993 from 923 to an
estimated 1,095 (Koopman 1993; Simmons, pers.
comm.). This is an 18% increase. These taxonomic
changes have implications for preconceived geographic
ranges. Older published maps and museum identifications
may no longer be valid (Simmons, pers. comm.).
For several families (Emballonuridae, Noctilionidae,
Mormoopidae), acoustic identification of species
appears to be straightforward (O’Farrell and Miller 1997,
1999). Others (e.g., Natalidae, Thyropteridae, Vespertilionidae,
Molossidae) are more problematic. As my
field techniques were refined and the limits of the equipment
established, it was clear that acoustic identification
in the field was not an option for species from two families.
These were Natalidae, the funnel-eared bats, and
Thyropteridae, the disc-winged bats.
The Mexican funnel-eared bat (Natalus stramineus)
appears to be a gleaner that is too quiet to detect in the
field. This species is readily trapped and occasionally
mist netted. A large number of fragmented recordings
for a few individuals (n=24) were obtained, but only at
very close range. Rydell et al. (2002) also found this to
be the case when recording a single released individual
in Mexico using a time-expansion system.
Spix’s disc-winged bat (Thryoptera tricolor) is the only
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