Friday, 16 September 2011

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.

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