Bats are getting more and more popular and over 50 keen bat-watchers came on the Belair Park walkabout organised by the Dulwich Society and the Friends of Belair in July. And even though hardly any of us caught a glimpse of a bat as it ducked and weaved through the evening air, chasing its insectivorous supper, there were triumphant cries from trackers of all ages as three varieties of this small flying mammal revealed their presence.
Bat-spotting triumph came thanks to the wizardry of the hand-held ultrasonic detectors that Colin Higgins (from London Wildlife Trust) had distributed before he led the walk. By pre-tuning one of these battery-powered devices to a particular frequency - bat calls are so high that they are inaudible to all but the most sensitive of (young) human ears - it is possible to identify which species of bat is “shouting” in the vicinity. Its whereabouts can be pinpointed, even though it may be hidden from view by dimming light or thick leaf cover.
A bat emits a rapid series of squeaks as it flies. These extremely high-pitched sounds can be picked up and transmitted by the detectors, which look like small transistor radios, or walkie-talkies. They reach the human ear, translated into a stream of metallic clicks. A particular variety of bat can be identified by twiddling the detector’s knob before setting out on a walk, thus pre-setting the equipment to a particular “waveband” or frequency range, which is known to be that bat’s type. Each kind of bat will be “broadcasting” within a specific sound band.
That warm Thursday night, as dusk fell, the calls of two of the three Pipistrelle varieties most usually found in London, as well as the sounds of a Noctule, were picked up, loud and clear. (Members of the London Bat Group, who came along with their own state-of-the-art electronic detection equipment were particularly pleased by the Noctule because they had not realised they were using this particular Dulwich habitat).
Those of us who strolled back beside the lake were also able to see, as well as record on our borrowed handsets, a Daubenton’s bat swooping low, as the water reflected what was left of the fading light. A pipistrelle must eat up to 3,500 tiny flying insects in a single night (that’s a third of its bodyweight) if it is to find the strength to fly and hunt the next evening. In Belair, the railway line and embankment and the mature trees around the edges of the sports fields and lake are regularly used bat roadways. The newly planted Woodland Walk, edging the sports fields, aims to enhance this ideal insect-hunting ground.
Bat-detector equipment, like that used on the walk in July, is brilliant but it’s not always able to differentiate between some of the less-common species. Certain “pulses” of clicks appear so similar, between species, or are issued at such long or irregular intervals, that it would take an experienced bat expert to interpret them correctly. It is only by using a mist net to temporarily trap bats as they exit a roost, and examining them closely in-hand (an enterprise which is against the law unless carried out by a licensed bat handler) that species can be identified beyond doubt. At a distance, in the dark, bats may all look alike to us. But seen close-up, they do have very distinctive face and ear shapes. It was this netting method that proved the presence of a Brown Long-eared Bat (and probable presence of a Natterer’s) both previously unrecorded in Southwark, at Sydenham Hill Woods nature reserve in 2006.
In Belair, a number of batboxes (including a maternity roost) have been donated to the park by the Dulwich Society to try to help bats thrive locally and it is hoped that these will be in place before the winter. During hibernation, bats must find frost-free, dry quarters if they are not to die during their seasonal torpor. They interrupt their “sleep” to move to a safer roost if there is any danger of their blood freezing.