Bats in Wanstead Park
One of the most popular events organised by the Wren Wildlife and Conservation Group are bat walks. Why people should want to go out to meet at sunset on a not-necessarily warm evening - possibly to get bitten by midges and mozzies into the bargain - to experience a creature that even if they do catch sight of they'll not really be able to see in any detail, may be a bit of a mystery.
I suspect that part of it is because the whole experience includes aspects that are less common now than once was the case. The bats themselves are perhaps slightly mysterious creatures, and much less commonly seen than would once have been the case. Even being out in the open at night, other than in a town with street-lights, is a somewhat rare experience for many. There is a sense of adventure - hunting down a strange creature – and perhaps wonder, that such creatures may be seen locally.
Heronry Pond at sunset - a prime bat locationWanstead Park is a good spot to see them – in fact it is probably one of the better spots in the whole of Epping Forest. To discover what species of bats we have here calls for some level of expertise, and some technology. Without being really expert, it is difficult to recognise bats whilst in flight – and a special license is required to handle them even if caught. We can't usually get a “handle” on a species as it flies by, and sometimes we may not even be able to see them. So we make use of an electronic device known as a bat detector. These may be purchased from specialist suppliers. They work in a way that in some ways is analogous to a radio receiver, picking up the signals that a bat emits and translating those signals into a sound that is audible to the human ear. Where they differ from radios is that whereas these pick up radio frequency (RF) signals, bat detectors pick up the sounds (audio frequencies) that bats make whilst flying. These sounds are known as echolocation. We can't normally hear those sounds as most are at frequencies inaudible to most human ears. (younger people may do better at this, as people often loose the abilities to hear those frequencies which do come within human hearing range with age)
So, armed with a bat detector or two, a goodly crowd may assemble for a bat walk, to get bitten, trip over tree-stumps, possibly get cold and wet. The offset of all this is the pleasure of being in the company of other people, to have a slightly different experience to the norm, and to experience bats.
Tim Harris has been asked by the Bat Conservation Trust to do a series of transects (a regular and formally plotted walk) to try to ascertain whether Nathusius' Pipistrelle is present in Wanstead Park. This species is one of three Pipistrelle bats in the UK, and is the least recorded. We know that we have good populations of the other two: Common Pipistrelle and Soprano Pipistrelle, and Tim and I have had some indication in recent years that the third may be present. The uncertainty lies in the fact that we can only distinguish between the species by the frequencies of their sonar emissions, which is what the bat detectors are receiving.
Until only a few years ago, in the 1990s, even the Soprano Pipistrelle was not known to exist. However, bat detectors will indicate what frequencies the sonar emissions are on and it was realised that some of the Pipistrelles' echolocation sounds were at a slightly higher frequency, and was indeed a separate species. The frequency at which Common “Pips” are best heard is about 45KHz , whilst the Soprano is at about 55KHz. Nathusius', however is at about 39KHz.
Last year in preparation for a transect, Tim and I walked out and timed a route around much of the Heronry Pond in Wanstead Park. This year, Tim obtained a bat detector and recorder which enables better results than simple model bat detectors, and together with Sharon Payne we set out at 19.40 on 9th September to start the transect. The weather was amiable, a quite warm evening though with a somewhat cooling breeze. The transect takes the form of a series of timed stops-and-walks between pre-determined points.
Frequency plot of possible Nathusius' Pipistrelle, Wanstead Park, 9th September 2012Particularly along the north edge of the lake, we had numbers of bat “passes”, audible on the detectors if not necessarily visible to the eye. There was a mix of the sounds and frequencies indication both Common and Soprano Pipistrelle, but with just a couple of brief and less determined catches of possible other frequencies and species. However, one of the facilities available to us this evening was the ability to record the whole of the proceedings in such a way that a broad frequency-band log of everything picked up by the microphone was recorded for later analysis.
As I have access to analysis software, a couple of days later Tim and I played the recordings into my computer, and confirmed the Common and Soprano species. The two results which were different to these were of a bat which was emitting closer to 40KHz, which is a possible recording of a Nathusius' Pipistrelle, and another nearer to 60KHz which may be one of the larger species of bat.
The results of this transect will be sent to the Bat Conservation Trust for analysis, and we are required to do another transect in September.
Paul Ferris, 14th September 2012