The river at night

Even on an overcast night there is still just enough light to make out the river as it flows inky black through the wood and on into town. It is the movement of the river that reveals its presence, the ever-changing nature of the surface film, which ripples and catches the eye. Other than the gentle murmurings of the river itself there is a reassuring stillness that envelops you, drawing you in with soporific ease. I don’t come down to the river that often at night unless I have dropped a car back at work after an evening talk. Rather than take the short route home, I sometimes follow the meander of the river through the woods and up to the bridges that mark the point at which the ancient trackway once forded the river.

I am not the only creature out at this late hour. Occasional sounds reveal other inhabitants pushing through the vegetation; muntjac startled at my presence, or small mammals flitting between the safety of their burrows and the patches of fruits or seeds on which they feed. It is the bats, however, that draw me here. I can just about make out their high-pitched calls as they make feeding passes up and down the river, presumably catching midges and moths as they go. More often than not, I will have my bat detector with me. Switching it on, the high-pitched calls are transformed into a pattern of beats and pulses that I can hear more clearly. Adjusting the dial across different frequencies I can make a stab at their identification.

From earlier visits, with more advanced detectors, I know that many of these bats will be Common and Soprano Pipistrelles, with good numbers of Daubenton’s Bat and the occasional Brown Long-eared. The Daubenton’s fascinate me, their series of echolocation calls reminiscent of a jazz drummer beating out a rhythm that slows and then accelerates with consummate ease. These small bats feed low over the water, typically between five and forty centimetres from the surface, taking insects from the water’s surface through the use of their feet and tail (contact of the prey with the tail membrane triggers a ‘strike’ with the feet). Since suitable prey are most readily recognised on calm water, these bats favour the more sheltered, slow moving parts of the river, away from surface vegetation and out of the wind. This particular stretch of the river, as it meanders through a block of alder woodland, seems ideal. It is only when I reach the bridge, with its solitary street light, that I can see as well as hear the bats and watch them as they skim the dark water for food.

Eyes and ears for bats

Returning from the office the other evening, I stopped on the Nun’s Bridges to scan the river. It was not quite dark and there was always the possibility that I might spot one of the local Otters. Needless to say, my luck was out and no dark shape was to be seen cutting it’s way across the surface of the slow-moving waterbody. However, other mammals were abroad and I was soon entranced, watching the dancing silhouettes of a dozen or so bats as they hawked for insects low above the water. This small cloud of tiny, delicate mammals, ‘chattered’ away as they circled too and fro to snatch midges and other small flies.

The chattering calls, whose intense high-pitched pulses I can still just about pick up with my hearing, are used to target prey and derive from a technique known as echolocation. This is really a highly developed form of sonar, the bat sending out short pulses of intense sound and then monitoring the returning echo with its sophisticated hearing to build up a ‘map’ of its surroundings. Although many people are bewildered by the apparent complexity of bat echolocation, to the bat it is just another component of its sensory armoury – much like how we might view our sense of sight or taste. Bats make and hear sounds in the same way as most mammals; the echolocation pulses are generated in the larynx and the resulting echoes are picked up by their ears. Admittedly, the larynx of a bat is proportionally bigger than our own, relative to body size, because the call has to carry a great deal of energy in order to produce a useful echo. Most bats emit the echolocation call through their mouth but there are species, like the horseshoe bats, where the sound is emitted through the nose.

Because the echolocation calls of bats often differ in their core frequency, it is possible to identify the different species by the pattern of the call and the frequencies over which it occurs. For example, the calls of Daubenton’s Bat start at about 85kHz and drop to about 32kHz, with a peak in intensity around 45kHz. Simple bat detectors, which convert the inaudible calls to a frequency we can hear, can help you split bats into rough groups but more complex detectors, coupled with computer software, are needed to separate the calls of species which are similar in their outputs.

The size and behaviour of the bats hawking over the river suggested that these were one of our pipistrelle species, although there may well have been a Daubenton’s Bat or two in with them. I will have to bring my bat detector down to the river in order to find out.