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A Lunar Eclipse Sheds Mild On a New Habits In Birds

As a group, swifts have recorded some impressive feats. The Common Swift, for example, remains the record holder for the longest uninterrupted flight, staying in the air for 10 months straight. Now scientists have added a remarkable distinction to the insect-eating group’s accolades. Not only do Black Swifts also stay aloft for up to eight months, but they ascend to almost three miles in the sky during moonlit nights. This pattern of high altitude flights, synchronized with the moon’s cycle, is the first record of such a behavior in birds.

“It’s a completely new discovery,” says Anders Hedenström, an avian ecologist at Lund University and lead author of the research.  

Tracking devices attached to Black Swifts revealed the birds flew at low altitudes during the day and spent more time soaring, conserving energy. Generally, the birds climbed to higher altitudes, around 5,000 feet, at night. But on nights during and surrounding a full moon, the birds ascended up to 13,000 to 16,000 feet early in the night before later settling to 6,500 feet for the remainder of the night. The data also showed that the birds flapped more actively during these moon-filled nights compared to when the moon was below the horizon, indicating that the swifts were doing something that required light to see.

This combination of data suggested that moonlight was directly influencing the birds’ nighttime behavior. But the scientists couldn’t be sure of the cause and effect—until they realized the loggers recorded a fortuitous event: a lunar eclipse on January 20, 2019, that effectively turned off the moonlight. As the moon passed through the Earth’s shadow, the tracked Black Swifts rapidly descended from their typical moonlit altitude of 13,000 feet. When the lunar eclipse ended, the swifts returned to the higher altitudes. “It shows clearly that light has an immediate influence on their flight altitude,” Hedenström says.

A graphic shows how the altitude of a bird in flight changes depending on whether it is day or night, and whether it is a full moon or new moon.
A Black Swift’s flight altitude changes depending on time of day or night—and whether the moon is full or new. Graphic: Courtesy of Anders Hedenström

Relatively unstudied until recently, Black Swifts are widely scattered across western North America, often breeding in inaccessible crevasses near flowing water or even behind waterfalls. “There just wasn’t much known about them,” says co-author Rob Sparks, a spatial ecologist at the Bird Conservancy of the Rockies. A decade ago, Sparks and a team of researchers at Bird Conservancy of the Rockies uncovered their wintering location in western Brazil, but whether they spent so much time aloft, like the Common Swift and other European swifts, was unknown.

The tracking study began as an attempt to answer that question. Sparks captured seven Black Swifts at their nesting sites in Colorado in 2018, equipping each bird with a device that could reveal the birds’ location, altitude, and activities around the clock. But first, Sparks had to recapture the birds to download the data. He caught six birds in 2019, and nabbed the last one in 2020, still wearing its logger. “That it contained two fully complete years of data—that was fantastic, a real bonus,” Hedenström says.

Sure enough, the data showed the Black Swifts spent many months airborne when they were not breeding. Although they did occasionally touch ground—one bird even rested continuously for 11 hours—the birds spent 99 percent of their non-breeding period in the air. Why swifts stay aloft all the time is still an unsolved question, says Felix Liechti, an ecologist at the Swiss Ornithological Institute who was not involved with the study. Their long, narrow wings and streamlined bodies make them super-efficient fliers, but they are still burning calories to do so.  “Being in the air might be more safe than in a tree,” Liechti says. “But flying is more expensive than sitting around.”

Similarly, why Black Swifts respond to moonlight so distinctly is still somewhat of a mystery. While birds may ascend to evade predators, the biologists say that going all the way to 13,000 feet would be overkill. And, during a full moon, the available light should be similar whether the bird is one mile or two miles above the earth’s surface. So why so high? Humid clouds and fog drifting above the rainforests of western Brazil could force swifts to climb to where it’s cloud-free. But their more vigorous flapping during these moonlit nights suggest that they use the extra light to actively hunt for flying insects in the air. “[The eclipse] confirmed that they are doing something up there where they are dependent on the light,” says study author Susanne Åkesson, director of Lund University’s Centre for Animal Movement Research.

Earlier research by Åkesson showed that European Nightjars behave similarly at night, taking advantage of moonlight smorgasbords of insects. But unlike the swifts, nightjars hunt at low altitudes. The swifts’ ascent and descent over the course of the night, Hedenström says, could mean the birds search for insects as they fly upward and then adjust their altitude to the highest densities of insects.

The researchers say that their next step is investigating what other aerial insectivores do at night, and studying the insects themselves. “We know very little about the distribution of insects in the air,” Liechti says. Based on the swifts’ behavior, the biologists suspect that insects also fly at high altitudes over the Amazon. Small airplanes can survey insects at low altitudes, but scientists are starting to use radar to study insect abundance in the night sky. Radar observations indicate that insect density is higher at lower altitudes, making it more surprising to find these swifts ascending so high. “It would be interesting to know whether the insects would be changing altitude or not, and what they’re doing up there,” Åkesson says.

Liechti speculates that smaller insects may inhabit lower altitudes and bigger insects, which provide more food for Black Swifts, hang out at higher altitudes. This would help explain the flight patterns they observed. “It just remains to go there and find out,” Hedenström says. Another mystery is how swifts are getting sleep while airborne for months at a time. The researchers plan to investigate this question, but the trackers needed for this sort of study can only be used on larger birds.

While the research relied on high-tech trackers attached to the Black Swifts, the work also yielded a lucky and low-tech finding: One of the tracked birds wore a metal band on its leg, which showed the bird was at least 17 years old. “That’s a really old swift,” Hedenström says. Common Swifts have been known to live beyond 20 years—quite old for most birds other than seabirds. The swifts’ aerial lifestyle likely helps them avoid predators and parasites like avian malaria. In the end, their unique behavior may be worth those extra calories. 

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