Leave bat roosts and roost entrances unlit

Supporting evidence from individual studies

1. A replicated, controlled study in 1970 of 11 bat colonies within buildings in Ontario, Canada (Laidlaw & Fenton 1971) found that when bat roosts were left unlit, numbers of big brown bats Eptesicus fuscus and little brown bats Myotis lucifugus increased, whereas numbers decreased when roosts were illuminated with artificial lights. The number of big brown bats at a roost that was left unlit increased by 97%, whereas numbers decreased by 41–96% at illuminated roosts. The same was true for little brown bats (unlit roost: 57% increase; illuminated roosts: 53–89% decrease). Two bat roosts (one big brown bat, one little brown bat) were left unlit, and nine roosts (six big brown bat, three little brown bat) were illuminated with artificial lights. All 11 bat roosts were in attics and contained nursery colonies. Three types of light were used: safety lamps (60 or 100-W incandescent bulbs), cool fluorescent lamps (40-W tubes) or spotlights (150 W). Emerging bats were counted at each of the 11 roosts during 46 nights in May–August 1970.

   Study and other actions tested

   Referenced paper

   Laidlaw G.W.J. & Fenton M.B. (1971) Control of nursery colony populations of bats by artificial light. Journal of Wildlife Management, 35, 843-846.

2. 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.

   Study and other actions tested

   Referenced paper

   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.

3. 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 bats 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 ferrumequinum 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.

   Study and other actions tested

   Referenced paper

   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.

4. A controlled study in 2012 at a church in Norfolk, UK (Zeale et al 2016) found that when bat roost entrances were left unlit more Natterer’s bats Myotis nattereri emerged than when entrances were illuminated with artificial light. Eleven bats emerged from the roost when the entrances were left unlit, whereas no bats emerged on the first night entrances were illuminated, and two bats emerged on the second night. On the third night, after the light was switched off, all 11 bats emerged. However, emergence times were reported to be earlier than those recorded prior to lighting (data not provided). A 400-W halogen lamp was placed 7.5 m below the roost and directed upwards to illuminate the roost entrances. In July–August 2012, the movements of 11 radio-tagged adult female Natterer’s bats were monitored during four nights with the roost left unlit, four nights with the light switched on and four nights after the light was switched off.

   Study and other actions tested

   Referenced paper

   Zeale M.R.K, Bennitt E., Newson S.E, Packman C., Browne W.J, Harris S., Jones G. & Stone E. (2016) Mitigating the impact of bats in historic churches: the response of Natterer’s bats Myotis nattereri to artificial roosts and deterrence. PLoS ONE, 11, e0146782.

5. 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 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%). 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. Other confounding effects, such as changes in habitat and food availability in the wider landscape, were not accounted for.

   Study and other actions tested

   Referenced paper

   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.