A total eclipse means the sun suddenly disappears at a time of day when it shouldn't — and that can have some surprising effects on animals' behaviors.

During previous eclipses, people around the globe have noticed a surreal silence as birds, bugs and animals interrupted their normal activities.

And researchers have observed all sorts of unusual things at zoos: Flamingos gathered in a circle around their babies to protect them. Giraffes began galloping around their enclosure. Swarms of birds large enough to appear on radar suddenly left the sky and roosted in trees. Gorillas marched to their dens, expecting their final meal of the day. The ancient Galapagos tortoises started mating.

With a full solar eclipse coming to a large swath of the United States on April 8, scientists are gearing up to observe animals at multiple zoos in the path of the totality, in part because they were so surprised at what they discovered in 2017 during the last total solar eclipse in the U.S.

“I thought it was going to be nonsense, I didn’t think animals were going to be affected. at all,” said Adam Hartstone-Rose, a professor of comparative anatomy at North Carolina State University in Raleigh, NC. “Clouds pass over and they don’t react. It was so ephemeral.”

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For the 2024 eclipse, scientists will be watching the birds and beasts – and inviting citizen scientists to offer their observations as well – as the celestial curtain falls.

Pets, too, are being included in the surveys, though veterinarians say overall the biggest effect on their behavior will not be the sudden darkness, but rather the behavior of the humans around them.

"They're going to react more to our reactions, our excitement and our anxiety than anything actually from the actual eclipse," said Dr. Rena Carlson, president of the American Veterinary Medical Association.

2017 eclipse provoked some odd animal behavior

To set up the study, Hartstone-Rose and his collaborators set more than 40 people to watch 12 animal exhibits at the Riverbanks Zoo and Garden in Columbia, South Carolina. They began a few days before the eclipse so they could gauge how much the animals' behavior changed.

It was a perfect place to do the experiment. At 2:41 pm on August 21, 2017 the greater Columbia, S.C., area experienced the longest period of totality of any city on the East Coast, slightly more than 2 minutes and 30 seconds of total darkness.

More than two-thirds of the animals exhibited changed behavior.

• Gorillas: “Marched all together to be let in for the evening and were kind of perplexed, like ‘Why aren’t you letting us in?’” Hartstone-Rose said.
• Flamingos: “All of the adults gathered around the perimeter and clumped around the juveniles and the babies,” he said.
• Giraffes: Some of the zoo's herd began running around their enclosure, only calming down when the sun came back.
• Bears: Couldn't care less. “One of them lifted his head during the eclipse, but they basically couldn’t be bothered to react,” he said.
• Galapagos tortoises: The Riverbanks zoo has a group of 35 of the slow-moving giants which can reach 900 pounds and live for more than 150 years. During the peak of the eclipse, they started breeding.

Scaling up for the 2024 eclipse

For 2024, Hartstone-Rose is scaling up animal observations across the country. There will be studies at multiple zoos in the path of the totality, which will allow a larger number of animals to be observed. He’ll be stationed at the Fort Worth Zoo.

Given what they found with the tortoises, he’s curious to see what the bonobos will do.

“They’re very sexual animals,” he said. “When they get stressed out, their reaction is to have sex. I’m very curious to see if they react by mating."

Bird fall silent, head to their roosts

Almost anyone who’s been outdoors during an eclipse notices two things – the eerily light and the sudden silencing of the birds and insects.

Animals simply don’t know what to do with eclipses because they’re not something their biology is adapted for, said Andrew Farnsworth, a visiting scientist at the Cornell University Lab of Ornithology who has studied bird behavior during eclipses.

“Eclipses aren’t common but they’re not rare and we humans can plan for them. But for short-lived animals that may not be tracking astronomy with math, it’s pretty unexpected.”

In 2017 he and a group of researchers used radar to see just what the birds were doing during the eclipse, a project they plan to expand during this year’s April eclipse.

Using 143 weather radar stations, they watched the behavior of several types of birds.

Daytime soaring birds, raptors such as turkey vultures and red-tailed hawks, tended to go to roost. They depend on the thermals off the sun’s energy to soar so when it gets dark they head down to the Earth because they expect the thermals to go away, he said.

Birds that forage for insects during flight, such as swallows and purple martins, also leave the sky.

They’re foraging for insects while they’re flying but when night comes there’s a decline in their food source and they can’t see as well, he said.

When Farnsworth and his collaborators watched radar images in 2017, the extent of birds’ behavior was surprising.

“For me, the take home from the 2017 eclipse was the magnitude of the response we saw,” he said. Their study shows huge clouds of birds that had been wheeling through the sky swooped down to the surface in numbers so large it showed up clearly on radar scans.

This year researchers plan on expanding the number of stations they are monitoring and expect to see the behavior across a much larger swath of the country, from Texas to Maine.

A call to all Americans to observe in April

To get the maximum number of eyes watching to see what animals do, there's also a citizen science project that will let people across the country gather information about what wild, farm and domestic animals do.

The project is called SolarEclipseSafari.org. Taking part won’t interfere with people’s eclipse experience but will allow scientists to collate information about how animals react both inside the totality and in areas where there’s only partial darkness.

“We’re hoping to have thousands watching,” he said.

A NASA-funded study will also listen for the distinct quiet that falls when the sun is eclipsed and animals fall silent. Americans will be invited to capture sound as part of the Eclipse Soundscape Project.

Go outside and pay attention

All the researchers USA TODAY interviewed had one piece of advice – the most important thing about an eclipse is to simply go outside and experience it. Don’t fiddle with phones and cameras or computers the whole time.

“Just listen and look,” said Farnsworth. “It’s easy to get wrapped up in the technology. It’s really important to do the observations with your eyes and ears and make the connection to nature that comes with that.”

The experience, as Hartstone-Rose learned, can be life-altering.

“It’s awe-inspiring,” he said.