Reducing Marine Light Impacts on Birds and Bats
- Marcus Griswold
- Jul 25
- 12 min read
By Marcus Griswold
When you drive out of a city at night, you probably notice more and more stars in the sky. This isn’t because the stars are introverts, it’s because humans have produced so much artificial light at night that we can’t even see the stars. Much like a spiderweb, this artificial light permeates from cities into rural areas through what is called skyglow.
According to the World Atlas of Artificial Night Sky Brightness, 80 percent of the world’s population lives under sky glow. In the United States and Europe, 99 percent of the public can’t experience a natural night anymore. What we often forget is that natural darkness has value in the same way that clean water, air, and soil have value, and when we lose this, both people and animals suffer.
We might imagine the vast ocean is safe from these bright lights, but the light from development on the coasts even infiltrates the nearby oceans. Globally, at water depths of 3 feet, nearly 73,000 miles of ocean life are exposed to light, decreasing only slightly to 63,000 square miles at 30 feet depth.
What we don’t know much about are the impacts of artificial light on wildlife farther out in the ocean at locations like oil platforms, offshore wind sites, and even along shipping routes. But as humans explore the ocean more, the impacts of light will likely increase, and scientists are finding ways to protect these super migrators before they are lost.
Bird and Bat Species Affected By Marine Light
Often when we think about the ocean, we are focused on animals that live in the water like whales, seals, fish, and other species. But did you know that birds and bats are common wildlife found far from the coast?
Birds can be extreme migrators. For example, arctic terns migrate 18,000 miles, bar-tailed godwits migrate 7,000 miles, and sooty shearwaters migrate 40,000 miles. Birds that may be encountered in the ocean include both marine birds and non-marine migratory birds.
Marine birds spend all or most of the year in the marine environment, with seabirds spending much of their life offshore. Other birds travel through offshore areas only during migration. For example, the U.S. Atlantic Coast and adjacent offshore waters are part of a major bird migration route known as the Atlantic Flyway. Over 200 species of waterbirds and terrestrial birds use this route to move between their breeding and wintering grounds. Birds from high-latitude regions of North America and Europe funnel into the Atlantic Flyway during migratory seasons. Examples of seabirds that are active at night while migrating, foraging, or returning to colonies and are directly affected by light include petrels, shearwaters, albatross, noddies, terns, and some penguin species.
While we often associate bats with caves and hollows of trees, they do in fact make it to the ocean. While offshore bat encounters are rare, there is evidence that bats travel over water during spring and fall migrations. Migratory tree bats have been observed crossing offshore waters, with surveys detecting individuals as far as 100 miles from shore. European bat species move between distinct summer and winter habitats, with some species embarking on long-distance migrations of 1,500 miles. Known long-distance migrants include Nathusius’ pipistrelle, the noctule, the parti-coloured bat, and Leisler’s bat. Bats are thought to fly at low height during long distance flights, but migrating bats may fly at heights above 3,000 feet.
Sources of Marine Lights
Sources of light pollution at sea include oil and gas platforms, shipping vessels, and light-enhanced fisheries, as well as coastal cities, ports, and harbors. Newer sources of light are offshore wind structures, marine energy generators, and there will likely be more if ocean seabed miners are given access to the exploration of the ocean bottom to search for critical minerals such as cobalt, commonly used in battery systems.
Shipping and light-enhanced fishing vessels emit light predominantly in a horizontal plane. Effects of artificial light from shipping include attraction and disorientation of birds and bats near the ships, but also the impacts of multiple ships along a route on large scale migration.
Studies of oil rigs using floodlighting indicated the lighted area extended more than 328 feet past the rig itself. Light spill such as this serves no purpose and increases ecological impacts.
Renewable energy developers are looking to the ocean for power sources more often now. With a goal of 45 gigawatts of installed offshore wind capacity increasing in the United States by 2035, the potential exists for increasing impacts on birds and bats. Offshore wind turbines and other offshore platforms use artificial light to aid vessels and aircraft in their navigation around these structures. Concentrations of migrating birds around offshore wind farms have led to thousands of dead birds found in single incidents. Marine energy, including wave energy converters, are of increasing interest. The structures associated with marine energy are smaller than those associated with offshore wind.
Effects of Marine Light
Light Wavelength
The impacts of light on wildlife depends on both the wavelength and the output. Lighting at sea can carry across long distances as the direct light paths are typically only hindered by the curvature of the Earth, making it potentially more damaging than light on land. The effects of direct light emissions are made worse by artificial skyglow.
Animals perceive light differently than humans. The spectrum of artificial light typically ranges from 350 to 800 nanometers, which corresponds well with the wavelength of maximum absorbance of the photoreceptors of birds and bats, meaning that it perfectly affects their vision -- and not in a good way.
White light has the greatest effect on seabirds as it contains all wavelengths of light. Bright white deck lights and spot lights on fishing vessels attract seabirds at night, particularly on nights with little moon light or low visibility. All seabirds are sensitive to the violet – blue region of the visible spectrum (380 - 440 nm). The eyes of the Black Noddy and Wedge-tailed Shearwaters are very sensitive to shorter wavelengths between 425 and 500 nm, likely due to the need to see underwater.
Not only can artificial light in the blue and violet wavelength damage the retina, but it can also affect behavior and development. Changes in melatonin production that can be caused by blue light can affect daily behaviors such as bird waking, foraging behaviour, and food intake and seasonal cues such as the timing of reproduction in animals, causing off-spring to be born during non-optimal environmental conditions. You may be familiar with blue light filters used on phones and computers due to the impacts of this light wavelength on our own melatonin levels.
Additionally, nocturnally migrating birds are disoriented by red and white lights because these may interfere with the birds’ magnetic compass, while blue and green lights cause the least disorientation.
Attraction and Disorientation
We have long known that birds and bats can be attracted by light. Humans used fire to attract seabirds to hunt them for food, and reports of collisions with lighthouses date back to 1880. More recently artificial light associated with the rapid urbanisation of coastal areas has been linked to increased seabird mortality.
With increasing construction of tall structures such as buildings and windmills, human-made obstacles have been affecting birds to a larger degree. High rates of fallout, or the collision of birds with structures, occurs in seabirds nesting adjacent to urban or developed areas and at sea where seabirds interact with offshore oil and gas platforms, where they are the likely cause of hundreds of thousands of bird deaths annually. Even in the case of shorter structures, excessive lighting can still contribute to disorientation of migratory species.
Artificial lighting can inhibit birds’ use of visual cues as well as magnetic ones when they are migrating. For birds, blinking lights are less of a problem than colored lights, and red light is less impactful than light of any other wavelengths. Gas flares from oil and gas platforms can also affect seabirds and they can become engulfed, combusting in the super-heated air above the flame. Hazard and navigation lights installed on all offshore wind turbines and offshore platforms attract some species of birds. Many seabirds exhibit nocturnal activity, in part to avoid other avian predators or to prey upon bioluminescent organisms that rise to the ocean surface at night. Nocturnally active seabirds may mistake these lights for bioluminescent prey or star patterns used for navigation.
Artificial light can disorient seabirds, causing collision, entrapment, stranding, grounding, and interference with navigation (being drawn off course from the usual migration route). These behavioural responses may cause injury and/or death. Birds may starve as a result of disruption to foraging, hampering their ability to prepare for breeding or migration. Seabirds, particularly petrel species, can be disoriented by lighting from ships and may land on the deck, from which they are unable to take off. The effect of artificial light may be exacerbated by moon phase, wind direction and strength, precipitation, cloud cover and the proximity of nesting sites or migrating sites to artificial light sources.
Fledglings are more affected by artificial lighting than adults due to the synchronised mass exodus of fledglings from their nesting sites. They can be affected by lights up to 10 miles away. Fledglings are more vulnerable due to the naivety of their first flight, the immature development of nerves in their eyes, and the potential connection between light and food. Emergence during darkness is believed to be a predator-avoidance strategy and artificial lighting may make the fledglings more vulnerable to predation. Artificial lights are thought to override the sea-finding cues provided by the moon and star light at the horizon. It is possible that fledglings that survive their offshore migration cannot imprint their natal colony, preventing them from returning to nest when they mature.
Bats also have the potential to be impacted by artificial light, though blinking lights generally do not attract them. Like many migratory birds, bats use magnetic senses for navigation, which could also be impacted by artificial light. On land, bats have been found to use artificial light sources, where their insect prey congregate, as foraging areas. Bats may also be attracted to the lights of offshore wind structures to feed. Illuminated offshore structures, including illuminated wind turbines, pose sensory traps to bats, as these lure them in, to exploit potential foraging opportunities or roost locations. Artificial light may attract insects, drawing bats in, leading to death or injury.
Changes in Prey Availability
Prey may change their behavior with increasing artificial light in the ocean. For example, most plankton move away from light, known as diel migration. They typically move to the surface in the largest synchronous movement of biomass in the world at night making for a yummy dinner for birds. Artificial light can affect plankton living as deep as 600 feet in the ocean. North Atlantic and Arctic copepods, Atlantic cod, and sea bream have been shown to avoid artificial light at night in the ocean, meaning less food for migrating birds.
On the other hand, artificial lights can also concentrate prey, which seabirds then take advantage of. Several gull species have been reported to increase their foraging opportunities on marine, coastal and terrestrial lit areas. For example, fishing vessels usually use light to concentrate fish and squid, which has helped Audouin's gull and Brown‐hooded gulls find their dinner easier.
Reducing Impacts of Marine Light
One of the most meaningful choices for lighting installations is to incorporate lighting only where necessary. This can be done through proper placement of light sources in addition to directing the light beams effectively. When deciding these factors, the purpose of the light should be clearly understood, while also knowing where unwanted light might cause harmful effects.
Several governments have developed recommendations and requirements for reducing light impacts from open ocean structures. The United States Fish and Wildlife Service and Department of the Interior Bureau of Ocean Energy Management Office of Renewable Energy Programs have published guidance for offshore wind facility lighting. The Australian Government has developed National Light Pollution Guidelines for Wildlife, including marine turtles, seabirds, and migratory shorebirds. Lighting standards for other types of renewable marine energy installations have been less widely issued to date, typically falling under broader categorizations for open ocean structures.
Six Steps to Assess and Reduce Artificial Light Impacts
There are generally six steps involved in assessing the potential effects of artificial light on wildlife and acting to reduce it.
Step 1: Describe the project lighting, including the location and size of the project footprint; the number and type of lights; their height, orientation, and hours of operation; site topography and proximity to wildlife and/or wildlife habitat.
Step 2: Describe the biology and ecology of wildlife in the area that may be affected by artificial light.
Step 3: Risk assessment of the potential impacts of the lighting design and operations on each species, based on their biology.
Step 4: Artificial light management plan to clearly document the risk assessment process, including the consequences that were considered, the likelihood of occurrence, and any assumptions that underpin the assessment. It should include how monitoring and auditing will assess the effectiveness of mitigation strategies used to reduce impacts to birds and bats.
Step 5: Biological and light monitoring and auditing should be used to determine the actual impacts on birds and bats, including monitoring before and after light installation.
Step 6: Use adaptive controls such as smart control technology for better controlled and targeted artificial light management. This can include remote management of lights, control of light color, dimmers, and artificial intelligence and cameras to detect species and modify lighting as needed.
Key Light Minimization Strategies
● Start with natural darkness and only add light for specific purposes;
● Use adaptive light controls to manage light timing, intensity, and color;
● Light only the object or area intended and avoiding siting lights in areas of concentrated migration;
● Use the lowest intensity lighting appropriate for the task;
● Use non‐reflective, dark‐colored surfaces;
● Use lights with reduced or filtered blue, violet, and ultraviolet wavelengths.
Light Location and Direction
Overall, the lighting of the water should be limited to minimize impacts on marine life, and bird-friendly lighting should be implemented with no upward emission, which will also limit contributions to skyglow.
The amount of lighting should be targeted to achieve minimum required or necessary light levels by reducing the number of lights or by moving from general area lighting to specifically focused task-based lighting. Light should not be allowed to spill out into the water or up into the sky unless for a particular purpose.
Light Intensity
Different light intensities are mandated by various regulations and are often given in units of luminous intensity such as candela, which is a measure of human-perceived light within a solid angle. In general, lighting should not be of higher intensity than the minimum requirements for a given purpose. In most cases, the effectiveness of lighting is not improved by making lighting brighter, and unintended side-effects are increased. Applying the minimum requirements for a given purpose may involve the use of controls to turn the light off when not needed or to have the option to dim the light. Ambient light sensors and/or timers can control both maintenance and safety lights, which need to switch on during low-light conditions and at nighttime.
Light Movement and Flashing
Most aviation obstruction and navigation lights have specified flash rates. Flashing lights have been shown to reduce bird fatalities and are often recommended as a bird-friendly lighting option. However, little research has been conducted about how flashing lights affect other marine life, so this area should be further studied. A near-infinite range of flashing frequencies, duty cycles, and on/off ramp times can be considered with modern LED lighting technology, and different cycles can be considered based on personnel needs, weather, and time of year (migratory) considerations.
Light Reflection, Transmission, and Scattering
Where possible, use non-reflective, dark coloured surfaces. Light reflected from highly polished, shiny or light-coloured surfaces such as white painted infrastructure, polished marble or white sand can contribute to sky glow. Light design will also need to consider actual light reflection due to ocean conditions and wave motion, fog, and weather. Testing temporary lighting in a variety of weather conditions should be completed before the final offshore designs are fully constructed.
Depth of Light Penetration
In marine lighting applications, light directed downward reflects but also has the likelihood to transmit into the water where it can affect aquatic life. Additionally, the entrance angle of the light into the water may affect how deep the light travels. As the light passes through the water, some wavelengths penetrate farther than others; short-wavelength visible light transmits the farthest in most conditions. Choosing light sources that have relatively low intensities of highly transmissive wavelengths can minimize how much light reaches the underwater environments.
Offshore Light Mitigation Toolbox
Australia developed a set of artificial light guidelines for wildlife that include the best practices below to reduce light impacts on birds and bats:
● turning lights off during the fledgling periods
● modification of light wavelengths
● removing external lights and closing window blinds to shield internal lights
● shielding the light source and preventing upward light spill
● reducing traffic speed limits and display of warning signs implementing a rescue program for grounded birds.
Additional mitigation measures listed, but not assessed for effectiveness, were:
● using rotating or flashing lights because research suggests that seabirds are less attracted to flashing lights than constant light
● keeping light intensity as low as possible. Most bird groundings are observed in very brightly lit areas
The guidelines also recommended against the use of certain lights such as mercury vapor, halogen, white fluorescent, metal halide, and white LED. They instead recommend the use of low or high pressure sodium vapor, filtered LED, filtered metal halide, or LED light appropriate for potential species present.
Citations
Australian Government. (2020) National Light Pollution Guidelines for Wildlife Including marine turtles, seabirds and migratory shorebirds. Available at: https://www.cms.int/sites/default/files/document/cms_cop13_res.13.5_annex_e.pdf
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Goodale, M. Wing et al. (2025). Opportunistic Vessel-Based Detections of Migratory Bats in the Gulf of Maine. Northeastern Naturalist 32(2), 168-178. https://doi.org/10.1656/045.032.0203
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Montevecchi, W. (2006). Influences of Artificial Light on Marine Birds In Ecological Consequences of Artificial Night Lighting (pp. 94-113). Island Press
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United States Fish and Wildlife Service: Threats to Birds: Offshore Structures. Accessed July 19, 2025. Available at: https://www.fws.gov/library/collections/threats-birds-offshore-structures
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Zhao, X., M. Zhang, X. Che, and Fasheng Zou. 2020. Blue light attracts nocturnally migrating birds. The Condor: Ornithological Applications, Volume 122, Issue 2. https://academic.oup.com/condor/article/122/2/duaa002/5780833
