How much notice do you take of artificial lighting during bat surveys? This information is just as important (if not more so) than weather data, such as temperature and humidity levels.
Artificial lighting has the potential to significantly affect bats, both directly and indirectly. Direct impacts include lighting inside bat roosts or onto roost access points. Some of these impacts are not always apparent, especially during daytime surveys. It is essential to make a judgement about artificial lighting, especially as it may be the most important limiting factor for bats. In the example below, even though there is extensive light spillage, bats can still (and do) roost in darker areas of the building.
Indirect impacts can include attracting bats’ insect prey into lit areas, which will mean less food for ‘light-shy’ bat species. It is surprising just how many insects can be attracted in this way – spending a few minutes beside a spotlight after dark is very instructive.
How much light does it take to cause a problem? It’s very tempting to look at the published guidance and apply this to all cases, but as we know every site and every situation is different (and therefore deserves individual consideration). For example, we have recorded lesser horseshoe bats feeding under streetlamps, and Natterer’s bats flying through spotlights.
Just as you should not use weather data from general sources when recording survey information (as it will not be specific to your survey site), you should not assume that lighting data from other schemes are applicable to your scheme. This is particularly the case for bats – there has been a tendency to rely on a few published sources as if they apply to all circumstances – this is definitely not the case and can be very misleading and unhelpful.
The most important and useful thing you can do is to measure the current lighting yourself, and to use this in your assessment. Measuring light levels is not difficult, but as usual, it is important to have suitable equipment and to use it in a way that makes sense; part of this means helping other people to understand what you have done.
You might find something surprising. For example, we have spent years measuring lux levels outside bat roosts at emergence times, and the lux levels were completely different to some of those in published documents. We have recorded lesser horseshoe bats emerging at over 140 lux and Daubenton’s bats emerging at over 20 lux.
In the same way, bat emergence times (which can be influenced by artificial as well as natural lighting) do not always fall comfortably within published guidelines.
We have recorded lesser horseshoe bats emerging 5 minutes before sunset, and common pipistrelle bats emerging over one hour before sunset.
If you don’t do so already, why not make a note to include a lighting assessment as part of your next bat survey? We guarantee that you will find it illuminating!
As always, if you would like our advice and support for any aspect of your work, including lighting surveys and assessments, please get in touch.
If you are involved in a development project, we can provide expert guidance and undertake bat and lighting surveys and assessments.
If you are an ecologist, we offer 1-1 and small group mentoring, as well as formal training, both face to face and via webinars.