Country diary: The night air is thick with bats

In the pitch-black night, Norway maples blaze orange on the far side of a blood-red lake. White bolts skim the ruddy surface of the water and flicker like St Elmo’s fire round our heads. It’s past 11pm, and we’re observing the scene on Hertfordshire and Middlesex Bat Group’s thermal-imaging cameras. My handheld monitor transforms the darkness into a multicoloured heatscape where common pipistrelles dart after midges and mosquitos, and Daubenton’s bats fly low over the lake – level-headed hovercrafts to the pips’ Hawk T1 jets.

Our bat detectors fill the silent night with a hard rock soundtrack that could have been composed by a morse code enthusiast. Daubenton’s bats rap out a rhythmic bassline with short bursts of staccato clicking. Above them, every pipistrelle has its own ultrasonic riff. With eight or more feeding simultaneously, they’ve adjusted the frequency of their echolocation calls (one proposed theory suggests this is to avoid confusion between individuals), and our detectors emit a syncopated chorus of slaps, crackles and pops.

We’ve heard five species tonight (my favourite is the quietest of all, the brown long-eared or “whispering” bat), but altogether nine of the UK’s 17 breeding bat species have been recorded here in Fairlands Valley Park, including rare species for the county such as barbastelle, Nathusius’ pipistrelle and serotine. One of the main reasons for such batty abundance is that once night falls, the 120-acre park becomes a dark sanctuary in this otherwise-illuminated town. Our aim this evening has been to set up transect surveys to provide data on how bats use the site, after the local council received a petition requesting “eco-friendly, wildlife-conscious lighting” along the park’s main pathways for safety reasons.

Studies show that artificial lighting can cause problems for bats, particularly light-averse species such as Daubenton’s, brown long-eared and barbastelle. Illuminating areas near a roost site can postpone, or even prevent, bats from emerging and, with nocturnal insect activity peaking at and just after dusk, a delay in feeding times can seriously affect bat health. This unlit park might confound our senses, but it enables lucifugous wildlife to feed, socialise and breed. Without the benefit of thermal sight and ultrasonic hearing, it’s easy to underestimate the importance of dark skies in a bat’s world.

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