24 August 2010. The nights are gradually drawing in and we start tonight’s bat walk a wee bit earlier, at 8:30pm. There is a strong and chilly wind blowing from the south-west and the cloud cover is almost complete – but it’s bright, following a pleasant sunny afternoon, and dry.
We have a small group assembled tonight, just three adults and three children. After the usual introduction we set off. A fox dashes across the road behind us ~ just a fleeting glimpse. We then hear our first bat at 8:58pm, a soprano pipistrelle. It takes a while before we see it. Only when the bat flies clear of the tree canopy and against the light sky does it comes into view. Soon there are others flitting about.
Pipistrelles are our smallest and most common species of bat in the UK. In fact we have two species of common pipistrelles that until just a few years ago were considered to be a single species. Bat detectors alerted bat workers to the realisation that some bats were echo-locating at about 45 kHz, while others were echo-locating at about 55 kHz. Further research revealed that the bats looked different – those calling at about 45 kHz had black faces and because of their resemblance to bandits, now tend to be called bandit pipistrelles! The others, because they ‘sing’ at a higher level, are commonly called soprano pipistrelles.
Further research revealed that these two species don’t just look different, and have distinguishably different calls, they behave differently as well. Sometimes the two species are encountered in the same general area but normally one would expect to encounter either one or the other, depending what type of habitat you find them in.
Of course, it is impossible to hear these echo-location calls without the aid of a bat detector. The electronic bat detector brings these calls down to a level that the human ear can appreciate. The high frequency calls emitted by the bat are bounced back off any objects in their path. Their brains compute the echoing sound and form it into an audio image of their surroundings. In this way bats can move about freely, and catch their food, even in total darkness.
Despite using high frequency calls, much higher pitched than human beings would ever be able to hear, bats do also make calls that we can hear – and they have eyes too! Anybody that has stood near a bat maternity roost at dusk on a summer evening will hear bats twittering and chirping in their excitement to leave the roost as soon as it is dark enough. But once bats are airborne, their high frequency echo-location system kicks in and they become silent to our ears, unless we use an electronic bat detector: then their secret world is suddenly revealed to us.
From 9:05pm onwards there are loads of bats about. The bat detectors are busy and the two young boys in particular are excited by the sight and sound of so many bats whizzing about in the twilight.
We encounter another swarm of feeding bats in the farmhouse garden. This is great. From a distance we investigate a bat roost in an old tree but we don’t see any bats emerge – they are probably all out and whizzing about by now. We hear feeding buzzes through the detectors. The bats are feeding – on midges!
It is time to move on. As we leave the garden, in the torchlight we spot a large garden snail being closely followed by a large leopard slug. The boys have never seen such things before and they are fascinated.
The full moon is rising higher into the sky as we make our way back through the farm to our starting point. We hear an occasional bat as we proceed but most of them are behind us now, still feeding in the vicinity of the large trees and old buildings. One of the black farm cats joins us now and seems very pleased to see us. We finally finish tonight’s walk at 9:45pm.