Ross Baker and I led a bat walk on what turned out to be a fine evening, starting from from Weybridge Station car park and walking round to the fishing lake that is (conveniently) just a few minutes walk from our house, just north-west of Brooklands Museum. Before starting we gave everyone a bat detector and a quick lesson on how to use it - which is very simple, although actually identifying the bats you hear is a bit more tricky. As I'm sure all our members know, bats navigate and detect prey using echolocation - a form of 'sonar' using high-frequency calls not normally audible to us. Bat detectors make these sounds audible, and by turning a dial until the sounds you hear are at their loudest and deepest, you can work out the frequency where the bat's call is loudest. That and the speed with which calls are emitted (they sound like a stream of clicks or chirps) help you to determine the species. However, of the 17 UK species, it is possible to identify only about half in this way with any certainty.
We started the walk around sunset as some species emerge very close to sunset, including the noctule (Nyctalus noctula), one of our largest bats (pictured below). These fly high and fast, looking rather swallow-like in flight; in fact because they emerge early they can sometimes be mistaken for these birds heading home to roost. We have often seen noctules flying over our garden, and sure enough started to hear their calls on the detectors and caught a brief glimpse of them flying overhead as we walked from the station towards the lake. These bats have a low-frequency call with a slow repetition rate - as a rule of thumb, the bigger the bat, the lower the frequency and repetition rate of the call. Low-frequency calls are also louder, so that it is possible to hear noctules even when they are visible only as a speck high in the sky.
Next, approaching the lake, we picked up calls of our two most common species, the common and soprano pipistrelle (Pipistrellus pipistrellus and P. pygmaeus), the latter imaginatively named because its call is loudest at a higher frequency. We had hoped to pick up Daubenton's bat (Myotis daubentonii) feeding over the lake itself, but the banks turned out to be so overgrown it was difficult to get a good view of the water. Moreover, the lake itself was so choked with weed it would not have attracted Daubenton's, which like to forage over open water. All was not lost, however, as we went back to the bridge across the River Wey adjacent to the lake, and there we did indeed get some fine views of these bats, aided by some spotlighting by Ross. This is about the only one of the Myotis species it is possible to identify from its flight pattern as it sticks very close to the water surface (rather like a small hovercraft), grabbing prey from the surface with its feet and tail membrane. All in all a pretty good night's bat-watching.
Lynn Whitfield