The total solar eclipse in North America on April 8, 2024 temporarily reset the biological clocks of some bird species, new research shows.
The eclipse created a dramatic spectacle as the moon moved between Earth and the sun, plunging midday into a few moments of total darkness along a path from Mazatlán, Mexico, to St. Johns in Newfoundland, Canada.
During and after the event, 29 species of birds sang as if a new day had begun, said the authors of a study published Thursday in the journal Science.
Many observations came from citizen scientists on the other side of the eclipse’s path, which helped broaden the scope of the study.
For scientists tracking animal behavior, the eclipse provided the perfect natural experiment to see how birds respond to temporary changes in light, which is difficult to achieve in the laboratory. According to the study, total solar eclipses typically occur in the same location every three to four centuries, meaning most wild birds have never experienced one.
Studying how wild animals respond to natural events like an eclipse provides insight into how they cope with rapid environmental changes and contribute to conservation efforts, said the study’s lead author, Liz Aguilar, a doctoral student in the evolution, ecology and behavior program at Indiana University in Bloomington.
“Light is one of the most powerful forces shaping bird behavior, and even a four-minute ‘night’ was enough for many species to behave as if it were morning again,” Aguilar wrote in an email. “This tells us how sensitive some birds are to changes in light, with clear implications for issues such as light pollution and artificial light at night.”
Just like humans, other animals draw inspiration from light for their daily activities, Aguilar said.
During the 2017 total solar eclipse, scientists observed the reactions of zoo animals, with some birds returning to evening roosts while nocturnal species became more active. But Aguilar and his team wanted to specifically document the responses of wild bird species.
The eclipse occurred during spring in North America, a crucial time when birds sing to attract mates, defend territories and migrate at night, she added. In spring, birds tend to vocalize more at dawn and dusk.
So what might happen to their daily and seasonal rhythms, already so strictly determined by light and darkness, if a brief night suddenly occurred during the day? For example, totality lasted four minutes and 15 seconds in Bloomington, Indiana.
After thinking about how to improve public interaction with the eclipse’s rarity and document animal behavior at the same time, the team designed a free smartphone app, Aguilar said.
“Community scientists have been absolutely essential to this project,” she said. “The eclipse traveled thousands of miles across North America in just a few hours, and our team couldn’t be in all of these locations at the same time.”
Nearly 11,000 observations from more than 1,700 users of an app called SolarBird, developed by researchers at Indiana University in Bloomington and Ohio Wesleyan University, captured bird behavior across 3,106 miles (5,000 kilometers) of the eclipse’s path before, during and after the event.
The team also installed cloth box-sized recorders in southern Indiana to capture about 100,000 bird vocalizations before, during and after totality, or times when the moon blocked sunlight entirely.
SolarBird users had a simple mission: spot a bird, observe it for a minimum amount of time during the eclipse, and document whether it sang, flew, or ate, among seven other behaviors.
“As we looked at the database that evening, we found that the community had also worked its magic,” said Dr. Paul Macklin, associate professor of intelligent systems engineering in the Luddy School of Informatics, Computing and Engineering at Indiana University in Bloomington.
The collected recordings were analyzed by BirdNET, an AI system capable of identifying bird species based on their calls. The team’s experts also analyzed bird song.
Of the 52 bird species active during the eclipse, 29 showed obvious changes in their vocalizations, the authors wrote in the study.
As the sky darkened, 11 bird species sang more than usual. In the darkness, some birds became quiet while others became more active.
But the biggest change occurred when sunlight returned, with 19 species erupting in what researchers called a “false dawn chorus.”
For example, barred owls called four times more often than they usually do. And robins, which have distinctive songs before dawn, sang six times their usual average.
For these birds, the return of sunlight marked the start of a new day, resetting their biological clock, Aguilar said.
“Different species of birds welcome the dawn in very different ways: some have loud, elaborate choruses, while others are much quieter,” Aguilar said. “We found that the species with the most intense choruses at dawn were also the most likely to respond to the eclipse.”
Although measuring long-term effects was beyond the scope of the study, the team believes that lasting impacts would have been minimal. However, the time taken by some birds to respond to the sudden darkness could have been spent searching for food, finding mates or defending their territory, Aguilar said.
Not all bird species reacted the same way, Aguilar added, and bird responses were strongest in the path of totality, where the moon completely covered the sun.
“It actually makes sense that not all species reacted the same way: Birds differ in their sensitivity to changes in light,” Aguilar said. “Each species has its own activity patterns, energy needs and sensory capabilities, so they interpret environmental changes differently.”
For some birds, observing others may even have made them realize that the darkness was not a real night, Aguilar added.
It was important for the authors to validate the BirdNET detections, said Dr. Stefan Kahl, creator and lead of BirdNET technology at Cornell University’s K. Lisa Yang Center for Conservation Bioacoustics. Although Kahl was not involved in this study, he has also participated in research related to bird behavior during the 2024 eclipse.
“BirdNET/AI makes mistakes, but these errors are systematic and repeatable, so I suspect we can see differences in vocal activity during an eclipse,” he said. “We know from our own study that not all birds respond the same way. The study authors report the same thing, so it seems reasonable.”
Dr. Andrew Farnsworth, a visiting scientist at the Center for Avian Population Studies at the Cornell Lab of Ornithology, separately studied bird behavior during the 2017 and 2024 eclipses. The new findings closely match what he observed, and he welcomed the innovative addition of crowdsourced information from across North America.
“They’re really useful in helping to understand how organisms perceive their environment, respond to their environment and provide us with some of the raw materials to understand how these behaviors evolve, are maintained and how they might change,” Farnsworth said.
Understanding which bird species respond to changes in light could be essential as the world becomes brighter due to the increasing presence of artificial light.
A recent study found that light pollution can disrupt time signals in birds, causing them to raise their voices for an hour longer than usual, especially birds with larger eyes or exposed nests. Light pollution can also disorient migratory birds, leading them to fatal collisions with buildings in large cities.
Fortunately, there is a growing movement to combat light pollution and reduce its impacts on various animal species.
“Knowing which species are most affected could help focus conservation efforts where they are most needed,” Aguilar said.
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