BioEd Online

When a team of researchers found hints six years ago that bats hunt migrating birds by night, some found the story hard to swallow. But a new study now confirms their suspicions.

The greater noctule bat (Nyctalus lasiopterus), the biggest bat in Europe, lives off insects in the summer. But in the autumn and spring, the bats turn their attentions to the huge flocks of songbirds that migrate at night to avoid daytime predators such as falcons.

When Carlos Ibañez of the Doñana Biological Station in Spain and his colleagues found feathers in the bats' droppings, they suggested that the bats had eaten birds¹. But other experts argued that the bats could have swallowed feathers floating in the air, confusing them with insects.

"I was critical because eating feathers is no proof of eating bird flesh," says biologist Raphael Arlettaz of the University of Bern in Switzerland. Arlettaz argued that birds were too large for the bats to take down. He was also puzzled by the lack of bones in the bat droppings. Ibanez's team suggested that the bats might eat only the breast meat and discard the rest.

Critics also found it hard to believe that the greater noctule, which catches and eats its quarry on the wing, could prey on birds in the same way. Bats usually wrap their prey in their wings and kill it with a bite. This works for insects, but it could compromise flight when the prey is a larger and more powerful bird.

Blood suckers

In the new study, Ibañez and his team measured the levels of certain chemicals in the bats' blood and compared them with the levels seen in its potential prey. In summer the bats' blood reflected a mainly insect-based diet; in autumn, the team found a bird-like signature.

Even Arlettaz has been won over. He joined the team when he heard their preliminary results, and is a co-author on the new paper, published in PLoS ONE². "The most virulent detractors can become the best proponents, when they are convinced," he says. "The giant noctules clearly assimilate bird flesh into their diet — traces of bird flesh can be found in bat tissues".

Nobody has seen a bat take down a bird in flight, because the migrating birds fly at altitudes of over 700 metres.

Migrating birds are an abundant source of food, points out Ana García Popa-Lisseanu, a co-author also at Doñana Biological Station. What is surprising, she says, is that up to now nothing was known to exploit this source at night.