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In January 2023, scientists attached tracking devices to eight grey plovers on the coast of the Wadden Sea off the Netherlands. The hope was to learn more about the birds’ yearly migration to breeding grounds in the Arctic. And all was going well until late May, when one of the birds started acting strangely.
“The first thing we noticed was a sudden change in direction,” said Michiel Boom, a migration ecologist at the University of Amsterdam.
While the rest of the plovers headed northwest, this bird started going southeast. In fact, the bird appeared to have rapidly descended before landing in a rock quarry — a very unusual destination for a grey plover.
Soon after, the bird’s tracker stopped moving. It had become clear that this was not a bird with a confused sense of direction: The plover had met its end in the talons of a predator.
Dr. Boom remembers becoming overwhelmed with disappointment.
“You put a lot of effort into putting these trackers on the birds, and you want to get information from the breeding grounds,” he said. “So at that point, we were just mostly sad about the fact that we lost one bird.”
But then a closer look at the data revealed something curious — whatever happened to the plover, the tracker’s GPS revealed that it happened at 9,455 feet above the ground. Starting from approximate sea level, that’s much higher than the altitude birds of prey are known to hunt at.
“That was the first moment where we started to be like, ‘Well, that’s actually pretty insane,’” Dr. Boom said.
Other clues kept pointing toward a high-altitude predation event.
For instance, the data also revealed that the plover sped up in the moments before it changed direction. It had most likely spotted its attacker and was unsuccessfully trying to get away. What’s more, upon recovering the plover’s remains and tracking band in Sweden, scientists spotted a peregrine falcon nest a mere 650 feet away. This gave the team a suspect in the murder mystery.
Piecing it together in a study published earlier this month in the journal Ecology, Dr. Boom and colleagues provided evidence for the highest-flying predation event ever recorded.
Dr. Boom cautioned that kills occur all the time on the sides of mountains or high-elevation plateaus. But as far as a fight for survival involving predator and prey nearly 10,000 feet up in the sky? That’s something rare.
The incident may also offer clues to solve a different mystery — why do migratory birds fly so high?
For instance, long-beaked wading birds known as great snipes have been clocked flying 23,000 feet above Earth’s surface. But scientists can’t say why the animals climb to such extreme heights. Similarly, great reed warblers fly about 7,800 feet above sea level during the night, but ascend as high as about 20,000 feet during the day. Could it be that the migratory birds are trying to avoid predators in the daylight?
Sissel Sjöberg, a behavioral ecologist at Lund University in Sweden who documented the great snipe’s and the great reed warbler’s great heights, said the new study is not proof positive that migratory birds are evading predators. She thinks high-flying migration routes are mainly so birds can avoid overheating as they travel.
But she said the study did highlight that “falcons might hunt higher than we have imagined, and this is something that needs further understanding.”
Graham Taylor, a biologist at the University of Oxford, said he was most surprised by how the study showed that predatory birds may benefit from spending the time and the energy required to ascend so high in search of prey. Still, he admitted it was possible the falcon had caught a lift on a rising thermal, which would reduce the effort needed to ascend.
Two other factors may make the climb worth it for falcons.
“From a predator perspective, if there are large flocks of migratory birds flying at these really high altitudes, it might actually be beneficial to climb that high and hunt,” Dr. Boom said.
He added that by catching prey so high, the predator may save energy by slowly swirling down to a safe spot where it can tuck in.
While the new study doesn’t solve the riddle of high-altitude migration, that one out of eight grey plovers got picked off by a predator shows that, even 10,000 feet up, the sky is a dangerous place.
“Though it might seem like a random event, with one plover ending up predated at high altitudes, it underlines what these falcons are capable of,” Dr. Sjöberg said.
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