Here at NWS, we are often asked to advise on the ecological impact of lighting on bats. Generally we recommend keeping lighting to a minimum and directing it away from any potential bat roosts or foraging and commuting areas because bats can be very sensitive to both the light itself and its consequent effects.
Illuminating bat roosts may cause bats to abandon their roost as the light delays their emergence and shortens the time available to them for foraging. A peak in insect abundance occurs soon after dusk, so this delayed emergence means they miss out on important feeding. Many of these night-flying insects that constitute the bat’s diet swarm around street lighting. However, certain species of bat, notably the rarer species, avoid street lights, meaning they miss out on this abundant food source.
Lighting can also cause barriers to commuting bats therefore isolating populations. It can also make them more vulnerable to predation. Kestrels (an avian predator to bats) have been observed hunting at night under the artificial lighting along motorways.
We now use a lux meter to measure light intensity during bat surveys. The result from the lux meter can help us advise on the impacts of installing lighting to an area and from this we can offer recommendations on how best to limit the effects.
Dr Emma Stone, a researcher at the University of Bristol is conducting ongoing research into the effects of lighting on bats. From Emma’s research, an overview and guidelines surrounding this issue have been produced by both Emma and the Bat Conservation Trust. However, as part of their Bats and the built environment series, the BCT has produced a condensed, user-friendly version of these guidelines: Bats and lighting in the UK.