Bat Detectors – A Beginner’s Guide for Orthopterists
M. Lee January 2004
A Recording Revolution
The use of bat detectors to record orthoptera by picking
up their ultrasound emissions, particularly from bush-crickets,
has been both a revolution and a revelation. The process of
recording has been speeded up. In just three months use of
a bat detector in 2003, records on the ERCCIS database for
the Speckled Bush-cricket have increased by 80%, and for the
Dark Bush-cricket by almost 25%. The Speckled Bush-cricket,
formerly considered local, has been found to be plentiful.
The detector is also a quick means to obtain evidence of absence
for a species. Such negative evidence is important for confirming
any future distribution increase of species like the Long-winged
Cone-head.
It is not just an aid for those whose hearing may have deteriorated,
as it will greatly increase the range at which even an easily
heard species like the Dark Bush-cricket can be detected.
This means you will hear them before they hear you and dive
into the foliage.
Magenta Bat Mark 2 in use
What is ultrasound?
A young child can hear sounds across a typical frequency
range from 20Hz to 20kHz. Those at higher frequencies are
known as ultrasounds. The stridulation of each orthopteran
species will cover a range of frequencies, with that of grasshoppers
being mostly within normal hearing. The output of bush-crickets
extends well into the ultrasound. For the Speckled Bush-cricket
the greatest part is in the ultrasound, and even those with
perfect hearing may find its stridulation inaudible. Your
ability to hear higher frequencies diminishes with age (more
quickly in men than women), so a middle-aged adult may only
hear frequencies up to 14kHz. That is why even our loudest
bush-crickets may become totally inaudible as you get older.
What does it cost?
Bat detectors can be found from just under £50, to
well over £2,000. Fortunately, the cheapest set, the
Magenta Bat Mark 2, is a more than adequate machine for orthoptera
recording (if you have a flair for electronics, you can even
save a further £14 by buying it as a self-assembly kit).
It is of simple design with just an on/off/gain (volume) control,
and a frequency control (from about 20kHz to 130kHz –
guide indications only). There is a loudspeaker, as well as
a socket for an earpiece or headset. The easily available
9 volt PP3 battery will last for a couple of months of normal
usage.
How is it used?
There are few references on orthoptera recording with bat
detectors, but they quote a specific frequency for each species.
For a beginner, this seemed to pose practical problems in
their use. If it was set to one frequency, would you miss
all the others? Would you have to keep twiddling up and down
the frequency scale to ensure that all the species were detected,
and how slowly did you have to move the dial? And what if
the insect just happened not to be calling when you were passing
through its frequency?
Fear not, dear reader, because it is nowhere near as critical
as that. Just set the frequency control knob to a middle setting
around 35-40kHz, with the volume control at about half, and
then slowly pan the detector around. If there is anything
about, you will hear it. Once heard, if you wish, you can
rotate the frequency dial until you find the maximum level.
Whilst this frequency may be critical for determining the
identity of bat species, it is not essential for orthoptera.
In most cases, the sound itself is suitably distinctive for
species confirmation. Unlike bats, orthoptera are relatively
sedentary whilst calling, and you can always home in on the
sound (the detector is quite directional) to see the stridulating
insect, or a female attracted to the call. This will obtain
that conclusive identification.
Background ultrasounds
When using the detector, there is a surprising amount of
background ultrasound, especially that caused by your own
movements. The loudest comes from walking on grass or leaves,
particularly the dry foliage found in late summer or autumn
fields. This can swamp out the fainter orthopteran sounds.
For this reason, it is best to stand still whilst scanning
your surroundings. The effective range of a detector is about
15 metres, so a suitable quick recording method is to stop
every 10 paces and scan. In quiet country lanes, continuous
recording may be possible by walking on the edge of the tarmac
(beware traffic!) rather than the grass verge. Treading on
a pebble can produce a sound similar to a Speckled Bush-cricket,
so ignore single, unrepeated, clicks. If that elusive sound
on the detector is keeping pace with you, and stops when you
do, it is probably coming from keys or coins moving in your
pockets.
There are other ultrasounds which may lead to some confusion.
An odd ‘stridulation’ on the Camel Trail, near
Wadebridge, turned out to be made by cycle wheels as they
sped past. When you are near overhead power cables, they can
give rise to buzzing sounds vaguely reminiscent of a species
of bush-cricket.
Effects of weather
Orthopterans are much more likely to be calling on a sunny
day. Even on a hot day, a sharp summer shower may stop them
calling altogether, and they may not resume for some considerable
time. On a showery day, it may be best not to rely on a detector
for confirmation of a species’ absence at a particular
site.
Species accounts
It is not easy to give an adequate written description of
an audible sound, it will always remain subjective. In order
to illustrate the ultrasound call further, song diagrams (plotting
the change in sound over time) are given for most of the species.
These diagrams are all from actual field recordings made during
2003.
In Cornwall we have only a small number of species, and
it will be relatively easy to learn the different sounds made.
CD of Orthoptera Ultrasounds
With the exception of the Woodland Grasshopper, the various
ultrasounds of all these species were digitally recorded in
2003. If you would like a CD of all these calls for reference,
please send a cheque (made payable to Malcolm Lee) for £2
to:-
Gullrock
Port Gaverne
Port Isaac
Cornwall
PL29 3SQ
e-mail: gullrock@ukonline.co.uk
Further Information on Bat Detectors
For those with access to the internet, there is some useful
information on the different types of bat detectors. The two
most helpful were the Bat Conservation Trust website at http://www.bats.org.uk/
and Dr. Dean A. Waters Leeds University website at http://www.biology.leeds.ac.uk/staff/dawa/bats/Detector.htm
Information on the Magenta Mark 2 Bat Detector is at http://www.magenta2000.co.uk/kits/861.htm.
At the end of July 2004, the price of the fully assembled
unit was £48.99, and the self-assembly kit was £34.99.
An additional £3.00 p&p for standard delivery, or
£6.99 for next day delivery, is payable. Magenta Electronics
can be contacted on 01283 565435.
If you are in North Cornwall, this bat detector is available
for purchase at the Visitor Centre in Boscastle. Before you
travel any distance to buy one from here, telephone 01840
250010 to check they are still in stock.
Technical Stuff
The ultrasounds were recorded on a Hewlett Packard iPAQ
1910 Pocket PC (held about 10cms from the detector’s
speaker), using ‘ProTone’ v1.42 recording software
developed by Pocco Software. The bit rate was 43Kbits/second,
with a sample rate of 22Khz. The Voice Activated Start, Auto
Stop, and Compression (cut), were all disabled. Under Audio
Settings, the Microphone Environment was set to Short range
Recording.
The song diagrams were produced by importing the resulting
WAV files into ‘Creative Wave Studio’ v4.21.02
sound editing software developed by Creative Technology Ltd,
and capturing the screen image. The image was cropped and
the scale lines added.
And finally...
It is a bat detector, so don’t forget to listen out
for the bats as well!
Top of page
Home
- Site map - About
ERCCIS - Recording - Geology
- Services - News
and events - Publications
- Projects - Help
wanted - Free downloads
- Related links
|
