Letting the light in- How lighting can impact bat roosting.

Last week I started going into more detail about how light pollution impacts bat behaviour, specifically foraging. I hope it allowed you to see the complexity of the issue and left you wanting to know more. If so, you’re in luck! Today I’ll be going over how artificial light can affect the roosting behaviour of bats and a few simple ways you can help.

Roosts are where bats shelter (often in groups) and can vary lots in structure, with tree hollows, building roofs and underground caves all being used. Locations change regularly depending on the time of day, season and purpose to suit their requirements (wouldn’t you if it were free?!) [1]. For example, the maternity roost, where females gather to give birth and raise their young, will be different to their hibernation roost, where they rest throughout winter [2].

Roosts are really important habitats for bats, so much so that they’re protected in the UK, meaning it’s an offence to disturb or destroy them [1]. Due to its negative impact on bats, artificial light that falls directly onto a roost is actually classed as a disturbance!

Photo credit: Lindsay Carrington Ecological Services

So how does light affect bat roosting?

Roosts located in illuminated areas can delay bat emergence and prolong its duration [1, 3]. For example, a study on the effects of illuminated buildings on house-dwelling Geoffroy’s bats (Myotis emarginatus) found that the majority of the colony didn’t emerge until full darkness, with earlier emergence only returning two days after the lights had been disconnected [3]. Not only does this show that light delays emergence times, but also that some species can be slow to recover. Delayed emergence negatively impacts bats through reducing their foraging times and causing them to miss peak prey abundance, meaning they gather less food and have to rely on suboptimal prey [4, 1].

So, what does this mean?

This can impact bats individually, as reduced fitness can make them more susceptible to other threats [1]. However, the whole colony can also be compromised with problems arising such as reduced reproductive success and juvenile growth rates [1, 3]. For example, a study found that juveniles in an illuminated maternity roost were significantly smaller than those in dark roosts [5]. This is because juveniles rely on their lactating mothers who cannot gather enough food when their emergence is delayed.

As well as being at a disadvantage to larger juveniles, their lower body mass reduces their hibernation success as they have less fat stored [3]. Considering bats’ long lives and slow reproduction, populations impacted by reduced juvenile growth can take a long time to recover and have serious conservation implications! Light can also impact hibernation, as bats have been known to avoid these roosts in lit areas, which can impact their ability to survive [4]. Additionally, light spilling onto a hibernation roost can wake bats up, meaning they are wasting unnecessary energy [4].

Photo credit: CyberKat/Shuttercock

Artificial lighting around roosts can also force bats to change which entrance/exit they use [1]. These may be suboptimal, impacting their habitat connectivity and increasing mortality rates. This is because alternative exits can lead bats into exposed areas such as open spaces, roads and ground level, where predation and collision risk are higher [1].

As well as effecting the survival abilities of bats, long-term and direct light exposure onto a roost can actually lead to bats becoming entombed or abandoning them [1, 4]. This can result in sudden drops in populations, for example, numbers of little brown bats (Myotis lucifugus) and big brown bats (Eptesicus fuscus) dropped by 53-89% and 41-96% after light installation inside a nursey roost [6]. But that’s not even the worst of it! Research found that direct illumination onto a maternity roost of Geoffroy’s bats caused all 1,000-1,200 females to abandon it [3]!

How can we help?

Light pollution can impact roosting in so many ways and it’s evident that it can have some really serious consequences. Lights should never spill directly onto a roost and ideally the areas surrounding them. If you’re aware of a bat roost near to your home, making sure there’s as little light as possible surrounding it is really important. An easy way to do this is by positioning your lights to face away from the site and if you’re able to, place something like dense vegetation between the roost and light to lower the intensity. You could also install a bat box in a safe area of your garden which does not become illuminated at night! As roosts are so important to bat conservation, making small changes to lighting around your home (especially outside) can be a big help!

Next week will be the last blog which focuses on the impacts of light pollution on bats, specifically commuting!

You can access information on this issue and find out how to help by following Blinded by the Night’s Facebook and Instagram pages. If you have any questions about the campaign or would like to get involved, please contact me via email at Beth2.Gerrard@live.uwe.ac.uk.

How to get involved:

Facebook: Blinded by the Night @blindedbythenightbristol

Instagram: @blindedbythenight_bats


  1. URL: https://cdn.bats.org.uk/pdf/Bats_and_Lighting_-_Overview_of_evidence_and_mitigation_-_2014_UPDATE.pdf
  2. URL: https://www.researchgate.net/publication/242604931_Importance_of_night_roosts_for_bat_conservation_Roosting_behaviour_of_the_lesser_horseshoe_bat_Rhinolophus_hipposideros
  3. URL: https://www.researchgate.net/publication/232683086_The_effects_of_the_illumination_of_buildings_on_house-dwelling_bats_and_its_conservation_consequences
  4. URL: https://www.researchgate.net/publication/272889669_Impacts_of_artificial_lighting_on_bats_A_review_of_challenges_and_solutions
  5. URL: https://www.sciencedirect.com/science/article/pii/S0006320702002987
  6. URL: https://www.jstor.org/stable/3799798?origin=crossref&seq=1