Jim's Bat Page
The purpose of this page is to whet your interest in bats. It is by no means a full and comprehensive treatment of the topic. If you need more information I suggest that you look at the books and web links that I have given as references.
Bat Conservation
In Great Britain, bats are protected by law. To handle bats, or even to photograph them in their roosts requires a licence from the Nature Conservancy Council. Without a licence all we can do is watch them and listen for them with a bat detector. Even then they should not be disturbed and any movement which can generated ultrasonic noise should be kept to a minimum.
The Bat Conservation Trust helps to conserve bats through education and study and can provide advice and help. Visit the Trust's web site for more information about bats and suppliers of bat detectors. The web page also contains interesting sound bites of common British bats.
Bats find their way around in the dark and locate food by calling out and listening for echoes from nearby objects. In this way bats can detect such things as trees, buildings, the ground, telephone wires and flying insects. The echolocation systems of bats enable them to navigate, also to detect and home in on prey. In flight they are able to avoid obstacles and other bats. They are able to determine the type, location , direction and speed of their prey from the echoes they receive. An almost immediate echo will be received from a nearby object and it will be relatively loud. The echoes from further objects will be quieter and will take longer to return.
The sounds sent out by bats are very much higher in frequency than those that can be detected by humans. We can hear sounds in the frequency range 20 to 20,000 Hz whilst bats operate between 20,000 and 150,000 Hz (20kHz to 150 kHz). This is known as ultrasonic sound. Typically, Pipistrelles use 40 - 50 kHz and Horseshoes use 80 -100kHz. Bat calls are very complex. They may have several different frequencies or notes, and vary in loudness.
The speed of sound through air is fairly constant and is equal to the frequency of the sound multiplied by its wavelength. This means that if the frequency goes up, the wavelength goes down. If the speed of sound is about 0.3 kilometres per second, a 20kHz sound has a wavelength of about 0.015 meters or 15mm. A sound at 150kHz has a wavelength of about 2 mm. This wavelength range between about 2mm and 15mm is also the approximate range of sizes of the insects they eat. The shorter the wavelength (or the higher frequency) used by the bats then the smaller the prey that can be detected. The shorter wavelengths give more accuracy in homing in on prey.
The loudness of the ultrasonic pulses also changes depending on whether the bat is searching for prey or capturing it. It is loudest during search becoming quieter during capture.
One would expect that the bats would fly around using the highest frequency so that they could navigate and avoid obstacles with greatest accuracy. Unfortunately the highest frequencies do not travel far compared with the lower frequencies (longer wavelengths). For this reason bats use different frequencies for different purposes. They use the lowest frequencies for travelling and searching for prey, changing to higher frequencies to home in as the distance to the prey shortens, then the highest frequencies at short range to contact and seize the prey. The sounds sent out by bats usually consist of a series of pulses of ultrasound which sweep down from a high frequency to a low frequency over a few thousands of a second. The time space between the pulses allows any echo to return and to be processed in the brain of the bat to give it information about its surroundings, and any prey that may be in range.
Some bats use pulses which do not change in frequency. It has been shown that these bats detect frequency changes in the echoes due to either their own movement, or the movement of their prey, or both. The frequency change is due to what is known as Doppler shift. This is the effect that occurs when a police car with its siren wailing passes you. The frequency of the sound you hear drops as the car passes you. It is higher as it approaches you and lower as it speeds away from you. The amount of shift in frequency depends on the speed of the car - the higher the speed then the greater the Doppler shift. If the car was stationary and you ran past it fast enough you would also hear a Doppler shift due to your motion. The frequency shift that a bat detects is a combination of the relative speeds and directions if itself and its prey.
Humans see by building up a picture in the brain from light reflected from surrounding objects entering the eye. The bat brain builds up a similar picture from reflected sound. In the case of a bat, the sound is arriving in pulses and the sound fed to the brain must be a bit like the flashing light that we see at a disco. Just as our brains can make out something of the inside of the disco and see the dancers, the bats brain can build up a picture of its surroundings and nearby prey from the sound pulses it receives. In addition, from the nature of the echoes receive, the bat is able to determine what sort of prey it is seeing. For example, from the Doppler shift and the changes in loudness of the echoes due to pulses reflected from insect wings the bat can detect the speed that the insect is flapping its wings and deduce the size of the prey. Small insects flap faster than large ones, for example, mosquitoes beat their wings more than 200 times a second whereas larger insects such as moths and large beetles may only beat about 50 times a second.
Larger bats generally use lower frequency ranges than smaller bats. This means that they can see further than smaller bats but do not see the smallest insects, - which they would find it hard to catch anyway.
The echolocation methods used by bats are good enough to allow capture of the prey in the bat's mouth most of the time. For near misses, the wingtips and tail are used to scoop up the prey.
Bat detectors are devices that pick up the ultrasonic signals sent out by bats, and that we cannot hear, and converts them to a range of sounds that we can hear. They are a bit like a radio receiver that picks up signals that we cannot hear and makes them audible to us through the loud speaker. Like radio receivers, bat detectors also have a tuning dial to enable the user to tune through the range of frequencies used by the various types of bats. A very simple bat detector would have no tuning dial but it would be tuned to 40 or 50 kHz because most types of bats send out signals in that range.
Bat detectors are available commercially in various degrees of complexity and a wide price range. Wide band detectors accept a very wide range of frequencies and are useful when several bat types are being observed without the need for re-tuning. Narrow band detectors are available which allow the user to tune in to a particular type of bat without interference from other types. More complicated detectors have a time expansion facility enabling the slowing down of the received pulses so that individual pulses can be heard. These, together with pulse counters are comparatively very costly and used by serious researchers.
If you have electronic skills it is possible to build your own narrow band detector. The following design works well and has a range of about 60 feet for Pipistrelles. Adding a reflector to the microphone (similar to a radar or satellite t.v. dish ) would extend the range of the detector. The layout and construction of the unit is not critical and can be constructed "dead-bug" style. The output of this particular design is intended to be fed into the antenna socket of a shortwave receiver operating at 15.84 MHz. There is nothing special about this frequency - I chose it because I had a suitable crystal available. Any frequency can be used if the appropriate crystal is fitted in the Colpitts oscillator. The bandwidth of the detector is determined by the bandwidth of the receiver. A receiver with a beat frequency oscillator (b.f.o.) allows the reception of signals in the amplitude modulated (a.m.) mode for normal bat detection and in the single sideband (s.s.b.) or continuous wave (c.w.) mode for receiving the doppler shift of the signals. Anyone requiring further details can contact me.
Click here for circuit diagram
Further reading
For further reading see:
"Bats" by Phil Richardson
published by Whittet Books (ISBN 0-905483-41-3)
"The Natural History of Hibernating Bats" by Roger Ransom
published by Christopher Helm (ISBN 0-7470-2802-8)
Both of these books contain further references to other books on bats.
