An international team led by the Doñana Biological Station (EBD-CSIC) in Spain and Aarhus University in Denmark has documented for the first time how the greater noctule bat (Nyctalus lasiopterus), Europe’s largest bat, is capable of locating, pursuing, capturing, and consuming small birds in flight during their nocturnal migrations. The study, published in Science, was made possible through advanced tracking devices and the Singular Scientific-Technical Infrastructure of the Doñana Biological Reserve (ICTS Doñana).

The bat research team at the Doñana Biological Station has been studying the greater noctule for years. This forest-dwelling species is particularly difficult to monitor. The researchers use “smart” artificial roosts installed in the Doñana Biological Reserve. Each bat is implanted with a subcutaneous microchip, which is detected by an antenna in each roost. This system logs their entries, stores the data, and can send alerts directly to the research team’s mobile phones.

Two decades of attempts

For nearly twenty years, the team suspected that this bat species did not feed solely on insects, unlike other European bats, but also on small birds. This was suggested by the presence of feathers in their droppings during the spring and autumn migration seasons.

“We knew that the greater noctule captures and consumes insects, its usual prey, in flight, so we assumed it did the same with birds—but we needed to prove it,” explains Carlos Ibáñez, a researcher at the Doñana Biological Station - CSIC. However, the hypothesis was met with scepticism in the scientific community: some birds weigh up to 50% of the bat’s weight, seemingly incompatible with flight.

Later analyses of stable isotopes and DNA from feathers confirmed that the bats do indeed consume birds, identifying up to 31 different species that migrate at night at high altitudes. While this supported the idea that bats were hunting birds in flight, the process had yet to be documented in detail.

To do so, the team employed a variety of methods: cameras aimed at the noctules’ roosts, military radar, ultrasound recorders on hot air balloons, GPS trackers… However, the main limitation was the inability to tag the bats with devices light enough to record such events due to their low body weight.

Finally, they used ultra-light electronic devices developed by Aarhus University scientists. These devices record sounds (including the echolocation calls bats use to hunt and navigate), acceleration, and altitude. The devices were attached to bats in the Doñana Biological Reserve, and the information could be retrieved when the bats returned to the smart roost boxes, which detected their microchips and allowed the team to collect the stored data.

Confirming that greater noctule bats hunt and consume birds in flight

“Listening to the recordings was fascinating—it felt like traveling alongside the greater noctule in flight,” says Elena Tena, lead author of the study and a researcher at the Doñana Biological Station – CSIC. “We could hear its wingbeats and the surrounding sounds, like frogs in the Doñana marshes.”

In total, 611 hunting interactions were recorded. Most were brief attacks aimed at insects, but on two occasions, the recordings showed a different pattern: the bats climbed above 400 meters, detected a prey, and dove, flapping their wings faster and harder. The continuous buzzing with short intervals suggested that the bats were pursuing the same prey throughout the chase.

In one case, at the end of the pursuit, 21 stress calls from a bird were recorded, followed by 23 minutes of continuous chewing while the bat maintained altitude and continued emitting echolocation calls. This confirmed that the prey had been captured and consumed in flight.

Tena recalls a particularly emotional moment: “After so many years of research, hearing the bird’s stress calls followed by abrupt silence and prolonged chewing was striking. While it evokes empathy for the prey, it is part of nature. We knew we had documented something extraordinary. For the team, it confirmed what we had been seeking for so long. I had to listen to it several times to fully grasp what we had recorded.”

Identifying the prey

The device was retrieved several days later, preventing DNA analysis of the prey from that night’s droppings. To identify the species, the team compared the alarm calls to a database of 19 bird species previously found in the noctule’s diet. The prey was identified as an European robin (Erithacus rubecula).

Additionally, DNA from greater noctules was found on passerine wings collected from the ground in areas where the bats are known to hunt during bird migration periods. This suggests that to facilitate handling and consumption of such large prey mid-flight, the bats begin by cutting the wings.

This discovery not only confirms that the greater noctule can hunt migratory birds in flight but also precisely describes its hunting strategy. Birds, in turn, exhibit defensive behaviors similar to those used against daytime predators like hawks: diving down to try to escape.

Conservation implications

“For now, we’ve only recorded two cases, so we don’t yet know how frequently or opportunistically the noctule uses this technique. Still, the finding unequivocally demonstrates that this hunting strategy exists,” explains Elena Tena.

Beyond its biological interest, this discovery has important conservation implications. The greater noctule is listed as vulnerable nationally and globally and faces threats such as habitat loss, displacement by invasive species, and mortality at wind farms. Understanding its ecology and hunting behavior is essential for designing effective conservation and management strategies.

 “Having a fully monitored colony of noctules with microchips in the Doñana Biological Reserve was fundamental in this finding. Without years of tagging and tracking and without the cutting-edge scientific infrastructure and high-quality facilities at the Reserve, this would not have been possible,” says Carlos Ibáñez.