Assessing the impact of artificial night lighting regulations designed to protect astronomical observatories on seabirds and bats

An experimental study in the Canary Islands found that lighting conditions permitted under the "Sky Law" regulations, designed to protect astronomical observations, had no significant impact on the behavior of breeding Cory's shearwaters or bats, with environmental factors like moonlight and seasonality proving to be stronger behavioral drivers.

The Gist

Imagine the night sky as a giant, silent library where astronomers are trying to read the stars, and nature's creatures are trying to navigate their own nightly journeys. For decades, we've known that turning on too many streetlights is like shining a flashlight directly into someone's eyes in a dark room—it ruins the view for the astronomers and confuses the animals.

To fix this, the Canary Islands created a special rulebook called the "Sky Law." Think of this law as a strict librarian who tells everyone: "You can use lights, but they must be dim, and they must be a warm, amber color (like a sunset), not a harsh, bright white." The goal was to protect the astronomers' view. But here's the big question: Does this "dim, amber" rule actually help the animals, or is it just a band-aid on a broken leg?

This paper is the story of a team of scientists who decided to test the librarian's rules in the real world.

The Experiment: A Light Show for Birds and Bats

The scientists set up a stage in a beautiful ravine in Tenerife, a place where seabirds (specifically Cory's Shearwaters) come to feed their babies at night, and where bats hunt for insects.

They installed special lights that could change their "personality" every night. Sometimes they were off (the control). Other times, they turned on different types of lights allowed by the law:

• The "Warm Amber" Lights: Low and high intensity (like a cozy campfire).
• The "Cool White" Lights: Low and high intensity (like a bright kitchen bulb, but still within the legal limits).

They watched the animals like detectives with high-tech tools:

1. For the Birds: They strapped tiny, solar-powered GPS backpacks to adult birds. These backpacks recorded exactly how the birds flew, how fast they went, and if they got confused or delayed. They also put up microphones to listen to the birds' calls.
2. For the Bats: They used special "bat detectors" that listen to the high-pitched squeaks (ultrasound) bats use to talk and hunt.

The Big Surprise: The Animals Didn't Care (Much)

The scientists expected the birds to fly in zig-zags, get lost, or stay silent when the bright white lights were on. They expected the bats to either swarm the lights (eating the bugs attracted to them) or hide in fear.

But that's not what happened.

It was as if the animals walked past the light show and said, "Meh, I've seen this before."

• The Birds: The seabirds flew straight to their nests, at the same speed, and made the same amount of noise, regardless of whether the lights were off, dim amber, or bright white. They didn't seem confused or scared.
• The Bats: The bats kept hunting and flying just as usual. The type of light didn't seem to change their behavior significantly.

So, What Did Change Their Minds?

If the lights didn't bother them, what did? The real "villains" in this story were nature's own variables:

• The Moon: Just like humans, the animals were more active or less active depending on how bright the moon was.
• The Season: As summer progressed, their activity changed naturally.
• The Year: Sometimes, just by chance, there were more birds or bats one year than the next.

Why Didn't the Lights Work? (The "Old Timer" Theory)

The scientists have a few theories about why the animals weren't fazed:

1. They've Seen It All: The study area is near a busy tourist town. These adult birds and bats have lived there for years. They might have become "desensitized," like a person who lives next to a train station and eventually stops hearing the noise. They've learned to ignore the artificial lights because they've been there so long.
2. The Lights Weren't "Wrong" Enough: Maybe the difference between the "amber" and "white" lights allowed by the law just wasn't big enough to trigger a panic response. It's like trying to scare a cat with a flashlight when it's already used to the TV being on.

The Takeaway: A Good Start, But Not the Whole Story

This study is like a reality check for the "Sky Law." It suggests that for adult animals living in already-light-polluted areas, the current rules might be doing a decent job, or at least, the animals have adapted.

However, the scientists warn us not to celebrate too early.

• The "Newbies" are at risk: The study looked at adult birds. Baby birds (fledglings) on their very first flight are much more likely to get confused and crash into buildings. We don't know if the law protects them.
• We need to test in the wild: To really know if the law works, we need to test it in pristine, dark forests where animals have never seen a streetlight before.

In short: The Canary Islands' "Sky Law" might be a good shield for the astronomers, and it seems the local adult wildlife has learned to live with the current lighting rules. But to truly protect nature, we need to make sure those rules work for the animals that haven't had a chance to get used to the lights yet.

Technical Summary

1. Problem Statement

Artificial Light at Night (ALAN) is a rapidly accelerating driver of global change, threatening biodiversity and astronomical observations. While regulations like the Canary Islands' "Sky Law" (Law 31/1988) were established primarily to protect astronomical observatories by controlling light intensity and spectral composition (specifically Correlated Color Temperature or CCT), their ecological effectiveness regarding wildlife has remained largely untested.

The study addresses a critical gap: Do lighting conditions permitted under these astronomically motivated regulations actually mitigate ecological impacts on sensitive nocturnal taxa? The researchers focused on two groups known to be highly sensitive to light pollution:

• Seabirds: Specifically breeding adult Cory's shearwaters (Calonectris borealis), which are prone to disorientation and attraction to lights.
• Bats: Insectivorous bat communities, whose foraging and commuting behaviors are often disrupted by artificial light.

2. Methodology

The study employed a controlled field experiment conducted in Barranco del Infierno, Tenerife (a Natura 2000 protected area), during July and August of 2023 and 2024.

Experimental Design:

• Light Treatments: Five conditions were applied randomly on alternating nights from sunset to midnight:
  1. Control (C): No light.
  2. Amber Low (AL): ~1800K PC amber, low intensity (9,280 lm).
  3. Amber High (AH): ~1800K PC amber, high intensity (18,560 lm).
  4. White Low (WL): ~2700K white LED, low intensity.
  5. White High (WH): ~2700K white LED, high intensity.
     Note: 2700K represents the most permissive CCT allowed under the Sky Law.

Data Collection:

• Seabirds (GPS Tracking): 26 breeding adults were fitted with solar-powered GPS loggers (OrniTrack-9). Researchers analyzed 211 inward flights to the colony, measuring:
  • Flight speed, turning angle, duration, and tortuosity.
  • Time since sunset (TSS) to arrival.
  • Nearest distance to light (NDL) and altitude at NDL.
  • Number of nightly visits (NV).
• Seabirds (Acoustics): Passive acoustic recorders monitored vocal activity rates (VAR) as a proxy for colony attendance and stress.
• Bats (Acoustics): Ultrasonic detectors (SM4BAT) recorded echolocation calls to quantify movement and feeding activity (feeding buzzes) for species including Hypsugo savii, Pipistrellus maderensis, and Tadarida teniotis.

Statistical Analysis:

• Generalized Linear Mixed Models (GLMMs) were used to evaluate treatment effects.
• Covariates: Moonlight intensity, temperature, Julian date (seasonality), year, and recorder location were included to control for environmental variability.
• Post-hoc tests: Tukey adjustments were used for multiple comparisons.

3. Key Contributions

• Policy Evaluation: This is one of the first experimental studies to empirically test the ecological adequacy of lighting regulations designed specifically for astronomy (the Sky Law) against biodiversity impacts.
• Multi-Taxa Approach: It simultaneously assesses the impact of light spectrum and intensity on both marine birds and terrestrial mammals (bats) in a real-world, semi-urban setting.
• High-Resolution Data: The use of high-frequency GPS tracking (1 fix/sec) combined with acoustic monitoring provides a granular view of behavioral responses that previous broad-scale studies may have missed.

4. Results

Contrary to the initial hypotheses, the study found no consistent or significant effects of the light treatments on the behavior of the studied taxa under the tested conditions.

• Seabirds (Cory's shearwater):
  • Flight Behavior: No significant differences were found in flight speed, turning angles, tortuosity, or arrival times across the five light treatments.
  • Vocal Activity: Vocalization rates were not significantly reduced by artificial light, even under high-intensity white light.
  • Drivers: Environmental variables (moonlight, temperature, Julian date) were stronger predictors of behavior than the light treatments.
• Bats:
  • Activity Levels: Light treatments did not significantly alter movement or feeding activity for the dominant species (H. savii and P. maderensis).
  • Species Specificity: Tadarida teniotis showed a non-significant trend of increased movement under low-intensity light and decreased activity under high intensity, but no feeding buzzes were detected for this species.
  • Drivers: Similar to seabirds, moonlight and seasonality (Julian date) were the primary drivers of bat activity. Year-to-year variation was also significant.

5. Significance and Discussion

Interpretation of Null Results:
The lack of significant behavioral response to the "most harmful" (White High) vs. "least harmful" (Amber Low) treatments allowed by the Sky Law suggests two primary possibilities:

1. Acclimatization: The study site is in a heavily light-polluted urban area (south-west Tenerife). Breeding adults may have become habituated to artificial light over their long lifespans, losing the sensitivity seen in fledglings or populations in pristine environments.
2. Insufficient Contrast: The spectral differences between 1800K and 2700K (both warm tones) may not be sufficient to trigger a behavioral response in these specific taxa, unlike the stark contrast between red and blue light often cited in literature.

Implications for Conservation and Policy:

• Regulatory Context: The findings suggest that the current Sky Law regulations, which permit up to 2700K, may have limited ecological impact on adult seabirds and bats in already disturbed environments.
• Caveats: The authors caution that these results apply to acclimatized adults in light-polluted areas. They do not rule out severe impacts on fledglings or in pristine habitats.
• Future Directions: The study highlights the need for research in pristine, non-light-polluted environments to establish baseline sensitivities. It also suggests that while the Sky Law is effective for astronomy, its ecological benefits for wildlife in urbanized zones may be less pronounced than hoped, or that the specific spectral range tested (1800K–2700K) is not the primary driver of disturbance for these species.

Conclusion:
While the "Sky Law" successfully protects astronomical observations, this study indicates that under the specific conditions tested (warm spectrum, urban-adjacent environment), the permitted lighting does not significantly alter the behavior of adult Cory's shearwaters or local bat communities. Environmental factors like moonlight remain the dominant drivers of nocturnal activity.