Action: Leave bat roosts and roost entrances unlit
Key messagesRead our guidance on Key messages before continuing
- Three studies evaluated the effects of leaving bat roosts and roost entrances unlit on bat populations. One study was in the UK, one in Hungary and one in Sweden.
COMMUNITY RESPONSE (0 STUDIES)
POPULATION RESPONSE (1 STUDY)
- Condition (1 study): One replicated, controlled study in Hungary found that juvenile bats had a higher body mass and greater forearm length at unlit roosts than at roosts with artificial lighting.
USAGE (3 STUDIES)
- Use (1 study): One replicated, before-and-after study in Sweden found that all of 13 unlit churches continued to be used by brown long-eared bat colonies over 25 years, but bat colonies abandoned their roosts at 14 of 23 churches that were either partly or fully lit with floodlights.
- Behaviour change (2 studies): Two replicated, controlled studies in the UK and Hungary found that more bats emerged, and bats emerged earlier and foraged for shorter periods, when roosts were left unlit than when they had artificial lighting.
Lighting in the vicinity of a bat roost may cause disturbance, altered behaviour and abandonment.
Supporting evidence from individual studies
A replicated, controlled study in 2000 at two bat roosts within buildings in Aberdeenshire, UK (Downs et al 2003) found that when roosts were left unlit more soprano pipistrelles Pipistrellus pygmaeus emerged than when roosts were illuminated with white or blue lights at both roosts, or red lights at one of two roosts. More soprano pipistrelles emerged when both roosts were left unlit (average 40 and 90 bats) than when roosts were illuminated with white light (2 and 24 bats) or blue light (6 and 62 bats). Red light only had an effect at one of two roosts. More bats emerged at one roost when it was unlit (40 bats) than when it was illuminated with red light (13 bats), but at the second roost similar numbers emerged with (72 bats) and without red light (90 bats). A hand-held halogen light with coloured filters was placed within 3–5 m of each of the two roosts. Over 20 nights in July–August 2000, nights with roosts unlit and nights with lighting were alternated. On nights with lighting, white, blue and red lights were rotated in a random order and changed every 30 seconds. On each of 20 nights, the number of bats emerging per 30 second interval was counted at dusk.
A replicated, controlled study in 2003–2006 at nine buildings in north and south-east Hungary (Boldogh et al 2007) found that three bat species departed from roosts earlier and over a shorter period and juveniles were larger at roosts without artificial lighting. Lesser mouse-eared bat Myotis oxygnathus emerged between 21:10 and 22:15 at an unlit roost, compared to between 21:15 and 23:00 at an illuminated roost at which lights were turned off at 22:00 (over half the bats emerged after that time). Greater horseshoe bats Rhinolophus ferrummequinum showed a similar pattern. Geoffroy's bats Myotis emarginatus emerged between 21:00 and 22:00 at an unlit roost, but only after lights were switched off at 23:30 at an illuminated roost. The forearm length of juvenile bats was greater at unlit roosts (Geoffroy's bat: 36 mm; mouse-eared bat: 46–57 mm) than illuminated roosts (Geoffroy's bat: 31 mm; mouse-eared bat: 37–57 mm). Body mass of juveniles was also greater at unlit roosts (Geoffroy's bat: 6 g; mouse-eared bat: 15–23 g) than illuminated roosts (Geoffroy's bat: 5 g; mouse-eared bat: 11–20 g). The timing of emergence was measured (1–3 times) at two buildings when illuminated and when unlit, and at one unlit building. Body mass and forearm length of juvenile bats were measured at five illuminated buildings (133 bats) and three unlit buildings with similar conditions (same type of roof, 108 bats). Experiments were carried out in June–August 2003, 2005 and 2006.
A replicated, before-and-after study in 1980–2016 of 36 rural churches in southwestern Sweden (Rydell et al 2017) found that all of 13 unlit churches continued to be used by brown long-eared bat Plecotus auritus colonies over 25 years, but bat colonies abandoned their roosts at 14 of 23 churches that were either partly or fully lit with floodlights. Unlit churches continued to be used by significantly more bat colonies (13 of 13, 100%) than partly lit churches (7 of 13 bat colonies, 54%) or fully lit churches (2 of 10 bat colonies, 20%). Significantly fewer bat colonies abandoned their roosts at partly lit churches (6 of 13, 46%) than at fully lit churches (8 of 10, 80%). All 36 churches were surveyed during one daytime visit in summer between 1980 and 1990 before lights were installed. Floodlights (1–4 lights) were installed on 23 churches (date of installation not reported). Lights were directed upwards illuminating the walls and tower of each church either on one side (partly lit, 13 churches) or from all directions (fully lit, 10 churches). Thirteen churches were left unlit. Surveys were repeated at each of 36 churches in May–October 2016 after lighting had been installed.
- Downs N.C., Beaton V., Guest J., Polanski J., Robinson S.L. & Racey P.A. (2003) The effects of illuminating the roost entrance on the emergence behaviour of Pipistrellus pygmaeus. Biological Conservation, 111, 247-252
- Boldogh S., Dobrosi D. & Samu P. (2007) The effects of the illumination of buildings on house-dwelling bats and its conservation consequences. Acta Chiropterologica, 9, 527-534
- Rydell J., Eklöf J. & Sánchez-Navarro S. (2017) Age of enlightenment: long-term effects of outdoor aesthetic lights on bats in churches. Royal Society Open Science, 4, 161077