Featured Image Caption: Light pollution is a worldwide issue, especially in urbanized areas. This threatens a variety of species, from insects to turtles to birds, including those which navigate using moonlight. (Credit: Data: Marc Imhoff/NASA GSFC, Christopher Elvidge/NOAA NGDC; Image: Craig Mayhew and Robert Simmon/NASA GSFC, in the public domain, via Wikimedia Commons.)
Citation: Rodríguez A, Rodríguez B, Acosta Y and Negro JJ (2022) Tracking Flights to Investigate Seabird Mortality Induced by Artificial Lights. Front. Ecol. Evol. 9:786557. doi: 10.3389/fevo.2021.786557
By the light of the stars
What is your go-to method for driving to a destination you’ve never been to? While humans have tools — GPS systems, maps, and compasses — to help us find our way, many animals navigate using lights in the night sky. For example, many migrating birds orient themselves using the stars. Sea turtle hatchlings use the moon’s reflection on the water to figure out the direction of the ocean. Even dung beetles keep themselves oriented using the Milky Way.
This amazing ability to navigate can be compromised by artificial night lighting. Stars are drowned out when non-natural lights brighten the sky (a phenomenon called “light pollution”). Many animals are attracted to bright light sources, which not only draw them away from their intended destinations, but may cause them to circle the light to the point of exhaustion. Unfortunately, light pollution has become more prevalent with global urbanization, and it is more important than ever to understand how to protect wildlife from its most negative effects.
Petrels and shearwaters are seabirds often impacted by light pollution. Every year, petrel fledglings leave their nests for the first time and fly towards the ocean, but many are disorientated by artificial lights and are grounded before reaching it. This is incredibly dangerous for them because grounded birds are vulnerable to predation, dehydration, and malnutrition. Grounded birds may also be injured or killed through collisions with human structures. “Rescue campaigns” exist to collect grounded birds and release them back into the ocean, but some birds still die before collection or release.
Release the birds!
To better understand when birds are more likely to be grounded, researchers attached GPS data-loggers to Cory’s shearwater fledglings. The Cory’s shearwater is the most abundant seabird in the Canary Islands, where this study took place. Like many other petrels, it nests in underground burrows. This makes it difficult to access and attach trackers to fledglings, so the researchers used birds collected by rescue campaigns after being grounded. Instead of releasing these birds by the sea, they were released in an inland natural cave meant to simulate a burrow.
Similar experiments have been done in the past, but they required recapture of the tagged birds in order to retrieve the information. The GPS loggers in this experiment automatically sent data back to the researchers, so information could be gathered even from birds which successfully reached the sea. Environmental information was gathered for the birds’ flight paths, including the land altitude and irradiance (the amount of light) of nighttime lights at each recorded point. General environmental information (moon luminance, wind speed, and sunset time) and physical information about each bird (body mass and amount of down) were also recorded. The scientists hypothesized that a brighter moon would result in less groundings, because it would be easier for the shearwater fledglings to use it to navigate. Similarly, they hypothesized that birds which flew over areas with higher levels of light pollution would be more likely to be grounded.
Shedding light on the issue
In all, 277 fledglings were handled and released for this experiment over three years. Of these, 37 were grounded after release, with the percentage of grounded birds increasing from the first year to the third. The fledglings’ masses all decreased from release time to rescue time, highlighting the stress that being grounded puts on birds. Fledglings had an average initial mass of around 575 grams, and most birds’ masses decreased by 5 to 40 grams. The greatest decrease was 103 grams.
As expected, birds which flew over brightly lit areas had a significantly greater chance of being grounded. Birds that were grounded also tended to have slower and less straight flights. This makes sense, as when birds are drawn closer to the ground they may need to slow down to avoid collisions. Additionally, a winding flight pattern might be a sign of a bird changing its direction multiple times to find a place for an “emergency landing” due to disorientation or exhaustion. When analyzing the birds’ physical measurements, the scientists found that the amount of down (soft feathers that young birds are covered in) was the only factor that made it significantly more likely for a bird to be grounded. It is likely that down reduces how aerodynamic a bird is, making it harder to fly.
This study emphasized the harm light pollution can have on seabird fledglings, which would be even worse without rescue groups to re-release grounded birds. We need more research to understand how light affects other animals and mitigate its impact, but even now it is clear that measures should be taken to reduce artificial light levels. People can help by learning about light-sensitive wildlife in their area, such as migratory birds or young animals that navigate using natural lights. Advocacy for city-wide light reduction when these animals are most at-risk could make a major difference, but even turning off personal artificial lights is the first step towards helping wildlife reach their destinations.