Bat Walk Nothing quite matches the atmosphere of a night-time walk, particularly when you are searching for some of the most mysterious and misunderstood animals on the planet. With the expert guidance of a seasoned ecologist, Gareth Harris, a few wide-eyed local villagers – myself included – were able to experience a completely alien ecological quirk that has been on display every night across the world for almost 50 million years. Bats are highly sophisticated and criminally acrobatic animals, and they are specialist night-time hunters. They have developed a system called echolocation that allows them to determine the location and size of an object by emitting high-frequency ultrasonic “clicks” – through their mouths or noses – and listening to their unique echoes. 18 species of bat are found in Britain, all of which are insectivorous, and they all help control the otherwise overwhelming populations of mosquitos and moths that would spread diseases and decimate our crops. It is a criminal offense to disturb a wild bat (unless licensed to do so by the Statutory Nature Conservation Organisation) so these calls are one of the most effective ways of distinguishing between the different species. However, we can’t hear these calls, which is why one of the first things that Gareth did when we met at the windswept car park of Coombe Bissett Down nature reserve was hand us all a bat detector: the most commonly used form of detector, called a heterodyne, which transforms the ultrasonic calls of bats into sounds we can hear. Within half an hour of walking leisurely through the darkness, the static buzzing of our detectors erupted into rapid gunfire – a unique pattern of ‘claps’, ‘smacks’, ‘clops’ and ‘slaps’ – as a bat flew overhead. The differing patterns and frequencies of these ultrasonic pulses are unique to each species, and we were fortunate to encounter six. Pipistrelles (Pipistrellus sp.) By far the most common bat species in Britain are the common (P. pipistrellus) and soprano (P. pygmaeus) pipistrelles. They may be Britain’s smallest bats, roughly 20cm in wingspan and weighing as little as a 2p coin, but one individual can consume as many as 3,000 flies, mosquitos and midges in a night. It is these species that you’ll most likely see in your gardens, and we saw plenty on them moving from their roosting sites in Coombe Bissett to the insect-rich grassland of Coombe Bissett Down. The calls of the common and soprano pipistrelles are a series of irregular ‘wet claps’ and squeals that peak in frequency at 46.5 and 55.5 kHz, respectively. However, they can easily be distinguished from other bats by their exceptional manoeuvrability and their unpredictable twists and turns that they take just above head height. Pipistrelles are our most common bats, and they are incredibly acrobatic. Serotine (Eptesicus serotinus) The show stealers of the night were two serotine bats – one of Britain’s largest bats. They are fans of beetles, especially dung beetles, which they catch off the ground or through a method used by most bats called aerial hawking: catching prey with their tail membrane and feet. Serotines are usually the earliest to emerge after sunset, and they can be found in a broad range of habitats, from woodland and pastures to hedgerows and suburban areas. This species has a repertoire of relatively slow and irregular ‘smacks’ that are extremely loud on the heterodyne detector when tuned at its peak frequency of 32.8 kHz. They are broad-winged, slow flyers that circle around trees and over open areas at a height of 4-12 metres. The loud ‘smacking’ sounds of a serotine bat. The amount of insects in the air make it look like it is snowing. Myotis (Myotis) Next on the list are the notoriously difficult to identity mouse-ear bats, or myotis bats. The peak frequencies of all the myotis bats fall between 47 and 51 kHz, and their calls consist of ‘clicks’ that individually plummet from roughly 100 to 25 kHz within a few milliseconds – impossible to pick up with a manually tuned heterodyne detector. However, Gareth was able to use his real-time expansion detector, hooked up to an iPad, to analyse the structure of the sonograms (frequency against time) produced by the bats: whiskered bat (Myotis mystacinus) and Daubenton’s bat (Myotis daubentonii) were the most likely options. Both of these species are found in mosaics of woodland, agricultural areas and grasslands, however, the Daubenton’s bat is typically associated with slow-flowing freshwater; it is an active hunter of mayflies, crane flies and caddisflies, which it scoops from the water’s surface. Gareth explains to the group how tricky it is to identify myotis bats as a serotine continues to fly overhead. Barbastelle bat (Barbastella barbastellus) Those of you that have attended any of Gareth’s talks (bat-related or not) will most likely know that barbastelles are a favourite of his – and for good reason. This species is a specialist of small moths, which it catches by aerial hawking, but the barbastelle is also a specialist rooster: they are entirely dependent on the presence of old, dead trees. A single colony may occupy 30 old trees in the course of a season if there is a healthy supply of roosting sites and a structurally diverse understory. Unfortunately, large rotting oak and beech trees are becoming increasingly hard to come by, and the barbastelles are very sparsely distributed as a result. The echolocation of a Barbastelle is a unique, series of quiet, castanet-like ‘smacks’. They are fast, purposeful fliers but are particularly elusive as they typically only travel long after dark at about 4m above the tree crown. Gareth describes to the group the different sounds made by the different species. The one constant in almost all of Britain’s bats is that they like a mixture of woodland for roosting and insect-rich grassland for hunting. Habitat boundaries and corridors of trees and edges are incredibly important as very few bats will travel across an open space to find new feeding or roosting grounds. As a result, British bats are becoming increasingly isolated as urban development replaces once protective hedgerows, and the degradation of woodlands and grasslands reduces the quality of the habitat that they are trapped in. What most conservation organisations across the UK aim to do is increase connectivity; this means that species, from across the board, can migrate with changes in climate and move to new areas for feeding and breeding. Without those vital ‘hedgeroads’ – such that we are fortunate to have so much of in and around Coombe Bissett Down – we start to see the collapse in populations of some of our most important species. Bats are a major part of their ecosystems as well as our own livelihoods, and their services are utterly irreplaceable.