Artificial Lighting and Bat Friendly Design in Development Projects

Artificial lighting is an increasingly significant ecological constraint in development design. As lighting technology becomes more efficient and more widely used, previously dark areas can experience increased illumination with direct implications for bat behaviour and habitat connectivity. For project managers, designers, and contractors, understanding how lighting interacts with bat ecology is essential to ensuring lawful, proportionate, and deliverable mitigation.

This article outlines the scientific basis for lighting sensitivity in bats, the regulatory expectations in Scotland, and the practical measures that support bat-friendly design across development types.

1. Why artificial lighting affects bats

Bats rely on darkness to forage, commute, and access their roosts. Artificial light alters these behaviours by changing insect availability, altering predation risk, and influencing species-specific tolerance thresholds (BCT 2023). Research consistently shows that:

• Fast-flying, high-altitude species such as noctule and Leisler’s bat may tolerate some lighting, often feeding on insects attracted to lamps.

• Light-averse species such as brown long-eared bats and many Myotis species avoid lit areas entirely and may abandon flight paths or foraging routes in response (Stone et al. 2015).

• Roost emergence is delayed when entrances are illuminated, reducing feeding time and potentially affecting breeding success.

These responses apply at modest illuminance levels; even low-intensity lighting can create functional barriers.

2. Regulatory context in Scotland

While there is no standalone statutory instrument governing lighting impacts on bats, lighting is treated as a factor that can disturb bats or prevent access to roosts, both of which constitute licensable offences under the Conservation of Habitats and Species Regulations 2017 (as amended in Scotland) (Scottish Government 2019; NatureScot 2020).
Planning authorities frequently request lighting strategies where:

• roosts are present or likely;

• dark corridors form part of ecological networks;

• development risks severing key habitat connections;

• compensatory roosts require dark access routes.

NatureScot expects lighting proposals to be proportionate, evidence-led, and grounded in published guidance.

3. Assessing lighting risk during project design

Effective lighting design begins at concept stage. Key steps include:

3.1 Identifying baseline darkness

Understanding pre-development light levels is essential. Ecologists map existing lux levels, dark corridors, and sensitive features such as flight paths and roost entrances. Baseline measurements allow proportionate evaluation of impact magnitude.

3.2 Predicting lighting spill

Lighting designers should model horizontal and vertical lux levels using equipment specifications and mounting positions. Evidence shows that illuminance below 1 lux at sensitive features is a useful target, though thresholds must be species- and context-specific (BCT 2023).

3.3 Integrating ecological mitigation into design

Avoidance is the preferred approach. If avoidance is not possible, mitigation must demonstrate maintenance of access routes and avoidance of roost disturbance. Compensation should only be used where lighting impacts cannot be fully avoided or mitigated.

4. Practical design measures for bat-friendly lighting

4.1 Location and orientation

• Avoid lighting within 20–30 metres of known roost entrances where feasible.

• Direct luminaires away from linear features used as flight paths.

• Use shields, cowls, or baffles to restrict upward and lateral spill.

4.2 Lamp type and spectrum

Evidence indicates that warm-spectrum lighting (<3000 K) reduces ecological impact compared to blue-rich or broad-spectrum lamps (BCT 2023; Stone et al. 2015). Narrow-spectrum LEDs and filtered lamps may also reduce effects on insects and bat behaviour.

4.3 Intensity and duration

• Use the lowest illumination levels that meet safety requirements.

• Employ dimming, motion sensors, or curfews to restrict lighting when not required.

• Avoid constant illumination in sensitive areas.

4.4 Structural design integration

Positioning lighting on buildings rather than columns can help direct illumination downward and away from habitats. Conversely, lighting mounted close to eaves or soffits can inadvertently illuminate roost entrances and should be avoided.

5. Common misconceptions

Several recurring misunderstandings complicate design discussions:

“If bats feed around streetlights, lighting must be harmless.”
Only a subset of species exploit lit areas; many avoid them entirely, creating ecological fragmentation.

“Low-power LEDs do not affect bats.”
Light spectrum and directionality matter as much as intensity. Blue-white LEDs can create disproportionate impacts despite low wattage.

“Lighting can be reviewed after installation.”
Retrofitting is rarely effective. Impacts must be avoided through early design.

6. Integrating lighting design into ecological risk management

Lighting design should be informed by survey data and undertaken iteratively, with ecologists, lighting engineers, and architects collaborating throughout planning and detailed design. Where roosts, dark corridors, or potential licences are involved, early dialogue prevents late-stage redesign and reduces risk of non-compliance.

For licensing, NatureScot places strong emphasis on whether lighting proposals maintain functional access to roosts and preserve dark corridors required for Favourable Conservation Status (NatureScot 2020). Proposals lacking a clear lighting strategy often face determination delays.

Conclusion

Artificial lighting is a key ecological constraint in Scottish development, but one that is highly manageable through evidence-led design. Early assessment, interdisciplinary collaboration, and adherence to recognised guidance enable projects to proceed while maintaining functional ecological networks and avoiding licensable impacts. Thoughtful lighting design is both a legal necessity and a practical tool for reducing ecological risk.

References

Bat Conservation Trust 2023. Artificial Lighting and Wildlife: Interim Guidance. Bat Conservation Trust, London.
NatureScot 2020. Bats: Guidance for Professional Ecologists. NatureScot, Inverness.
Scottish Government 2019. Wildlife Crime and Protected Species. Scottish Government, Edinburgh.
Stone, E.L., Jones, G. and Harris, S. 2015. Impacts of artificial lighting on bats: a review of evidence. Mammalian Biology, 80: 213–219.