During spring and fall migrations, billions of birds take to the night skies. The high-altitude darkness protects them from predators.
Well, most predators. New research shows an unlikely creature can successfully hunt these migrating animals.
It’s a bat.
In a paper published Thursday in the journal Science, researchers detailed how the largest bat in Europe, the greater noctule bat, nabbed a European robin, before chewing and eating the prey in flight. This stands in stark contrast to the bats’ typical diet of insects.
The idea that these bats sometimes eat birds had already been inferred from indirect data collected by Carlos Ibáñez, a researcher with the Doñana Biological Station in Seville, Spain, and his colleagues. They had found clues such as feathers and bird DNA in the bats’ droppings. But until this paper, it remained unclear exactly how bats weighing around 50 grams — the size of a medium chicken egg — could catch birds approaching half their weight.
“We finally figured out the murder mystery,” said Laura Stidsholt, a bat researcher at Aarhus University in Denmark and an author of the study.
While bats are best known for hunting insects, some larger species can occasionally prey on frogs, lizards and fish. Tropical bats have also been known to snack on perching songbirds.
Dr. Ibáñez saw the indirect evidence of greater noctule bats eating birds in multiple studies over a period of decades. But he kept looking for more direct evidence of these high-altitude hunts, trying and failing to catch the bats in the act with military-grade radar and instruments lofted in balloons.
A turning point came when he and his colleagues, including the researcher Elena Tena, began collaborating with Dr. Stidsholt, who provided new lightweight sensors.
In spring 2023, the team outfitted 17 bats with these devices in Doñana National Park in Andalusia. At first, it seemed none of the bats were going after birds. But then one day, a tagged bat returned to a roost in the park.
The sensor’s audio revealed furious flapping, followed by the distress calls of a bird — and chewing.
The team combined the sensor’s audio, altitude and accelerometer data to recreate one bat’s impressive high-speed chase of what they determined was a European robin, based on the bird’s distinctive sounds.
After locating the robin, at around 4,000 feet up, the bat descended rapidly, following a characteristic path birds take when trying to evade falcons and hawks. The pursuit, Dr. Stidsholt said, was “like a dogfight.”
More than 3,000 feet into the dive, the bat caught and killed the bird, with a bite. The chewing sounds continued for 23 minutes as the bat remained flying.
In the paper, the researchers analyzed samples previously collected by Dr. Ibáñez near the roosts of greater noctule bats: wings of birds that bore the bite marks of the mammals. The researchers believe these bats rip off the wings of birds after catching them, to reduce drag. While feeding, the bats appear to avoid any bones.
This proof of hunting came as a culmination to a long career for Dr. Ibáñez, who recently retired.
Danilo Russo, a bat researcher at the University of Naples Federico II in Italy who wasn’t involved in the paper, called the study “a compelling example of how new technologies may help answer questions that have been waiting on researchers’ desks for many years.”
The authors say the findings help broaden our understanding of the bats’ diet and foraging behavior, which is especially important given the multiple threats to the species.
Although no visual evidence shows how the bats handle the birds, they most likely use their wing membrane and claws in a similar way to how they catch larger insects, Dr. Stidsholt said.
Catching birds is potentially risky, Dr. Tena said, but it’s effective for providing extra calories at crucial points in the bats’ life cycle: for gestation in spring and fattening up in fall before winter hibernation.
The researchers liken the ecology of the night sky to the deep sea. Even though many species of animals move through the expanse regularly, it remains barely known.
“It was very cool to have a peek into a very unstudied ecosystem,” Dr. Stidsholt said.
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