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How Does Outdoor Lighting Affect Bats?
A hedgerow can remain untouched, a line of trees can stay standing and a pond can look exactly as it did before a development.
Yet after dark, lighting can change how bats use all three.
For some species, illuminating a route between their roost and feeding grounds can make it a barrier. The habitat is still physically present, but it may no longer be accessible in the same way. Understanding this is an important part of designing outdoor lighting responsibly.
Why do bats need darkness?
Bats are nocturnal animals adapted to life in darkness. Moving and feeding after dark helps them avoid daytime predators. They use their surroundings in different ways. A building or tree may provide a roost, a pond may offer feeding opportunities, and a hedgerow may connect the two.
Artificial lighting can affect each of these activities. Its impact depends on the species present, how they use the site and where, when and how much light reaches their habitat.
What happens if a bat roost entrance is illuminated?
Light falling onto a roost entrance can delay bats leaving, or prevent them from emerging.
This matters because the period around dusk offers an important feeding opportunity, when insect activity can be particularly high. All UK bat species feed on insects, so losing that opportunity can affect their ability to obtain enough food.
The Bat Conservation Trust highlights potential consequences for survival and the growth of young bats. In some cases, lighting can cause bats to abandon a roost or become effectively trapped inside because they are reluctant to leave through an illuminated entrance. A light on a building can therefore have consequences beyond the area it was intended to illuminate.
What is a bat corridor?
A bat corridor is a route that bats use to move through the landscape, often connecting their roosts with feeding areas.
These routes can follow features such as
* Hedgerows and woodland edges.
* Lines of mature trees.
* Streams, rivers and other waterways.
* Connected gardens and green spaces.
Keeping these features in place is valuable. However, their usefulness to light-sensitive bats also depends on protecting the darkness around them.
What happens when we light a bat corridor?
For light-sensitive species, an illuminated section of a route can become a barrier.
Light spilling onto a hedge or waterway may cause bats to avoid it, disrupting their movement between places they need to reach. A feeding area may still exist beyond the lighting, but access to it can become more difficult.
This is why retaining a hedge on a landscape plan does not, by itself, establish that a bat corridor has been protected. The lighting design needs to be considered alongside it.
The question is not simply whether the habitat remains, but whether bats can continue to use it after dark.
How can lighting affect feeding?
Lighting can change both where bats hunt and where their insect prey gathers.
Some slower-flying, light-sensitive species avoid illuminated feeding areas. Meanwhile, insects attracted towards nearby lights may be drawn away from the darker places where those bats would normally feed.
The Bat Conservation Trust describes this redistribution of prey as a “vacuum effect”.
The result can be a combination of lost feeding habitat and reduced food availability in the areas that remain accessible.
But don’t some bats feed around streetlights?
Yes. Some faster-flying species can take advantage of insects gathering around certain lights. However, seeing bats near a lamp does not demonstrate that the lighting is harmless to bats generally. Different species respond differently. A location that attracts one species may exclude another. Feeding in an illuminated area can also expose bats to predators.
Observing bat activity around lighting therefore gives only part of the picture. Understanding which species are present, and how they use the wider site, is essential.
What should a lighting design consider?
The first step is to understand the ecological context. The Bat Conservation Trust identifies three important questions, which species are present, how they use the habitat, and what the existing light levels are. That information helps an ecologist and lighting designer establish where darkness needs to be protected and whether proposed lighting could affect a roost, route or feeding area.
The design process should then consider
* Whether lighting is necessary in the proposed location.
* Where light will fall, including spill beyond the intended area.
* The amount and distribution of light.
* When lighting will operate and how it will be controlled.
* The characteristics of the light source.
* How the installed scheme will be checked against the design.
No single fitting, colour temperature or control setting can establish that a scheme is appropriate for every bat habitat. Decisions need to respond to the species and circumstances of the site.
Why installation matters too
A considered design needs careful installation. Changing a fitting, altering its position or aiming it differently can change where the light falls. Controls also need to be set up correctly so that the lighting operates as intended. Where a design protects a dark corridor, those details form part of its purpose. They should be communicated clearly to the people installing and managing the scheme.
Darkness is part of the design
Outdoor lighting serves important purposes for people. Meeting those needs requires an understanding of the wider setting, including the wildlife that uses it after dark. For bats, darkness can provide access to shelter, food and connected habitat.
At Circalume, we consider the areas that need to remain dark alongside those that need illumination. Working with ecological advice, we help develop lighting proposals that respond to the site and its users. Because protecting a bat corridor means more than keeping the hedge.
Planning outdoor lighting near trees, hedgerows, water or a potential bat roost?
Contact Circalume to discuss how lighting design can work alongside your ecological advice.
Source and further reading
This article draws on the Bat Conservation Trust’s guidance, “Lighting Threats to bats”:
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