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RENEE MONTAGNE, Host:

Now is the time of year when billions of migratory songbirds fly north to their breeding grounds. Most of these birds fly at night, when there are no predators around, or so the experts thought until they heard about a giant European bat that plucks birds out of the sky.

NPR's John Nielsen has more.

JOHN NIELSEN: Not too long ago, a group of bat researchers walked into a marshy Spanish forest at dusk and spent the night making sound recordings. When those recordings were played back at full speed, the researchers heard a lot of croaking frogs and not much else. But when those recordings were played back at a tenth of their normal speed, the researchers heard, well, this.

(SOUNDBITE OF BATS CHIRPING)

NIELSEN: That's the radar call of a creature called the giant noctule bat, hairy and brown, with a wingspan slightly bigger than a blue jay and a huge mouth full of scary-looking teeth. It's the biggest and among the least-known bats in all of Europe, partly because it spends its days crawling around in the hollows found only at the tops of very tall trees.

ANA POPA: They are not so easy to see if you don't know that they are there, so most people don't know that they exist.

NIELSEN: Ana Popa-Lisseanu is a giant noctule expert at the Doñana Biological Station in Seville, Spain. She says it's long been known that these bats feed on flying insects, as many bats do.

What hasn't been known - at least until recently - is that the giant noctule may also be the only bat in the world that also eats birds on the wing. Popa-Lisseanu thinks it starts when the noctules fly thousands of feet up into the night sky. Then, after using those radar calls to lock in on migrating songbirds beneath them, they swoop in for the kill.

POPA: They wrap the prey between their wings, and between the tail membrane also, so they make like a kind of - of cage for the bird.

NIELSEN: When they fold their wings, these bats start falling like rocks, says Popa-Lisseanu. They also start eating as fast as they can.

POPA: They would just eat maybe the most profitable parts, like maybe the breast, for example, where the migrating birds accumulate all the fat and where they have the muscles.

NIELSEN: When the ground gets close, these bats open their wings and drop the mangled carcasses, or so it appears, since nobody's ever seen one of these meals on the fly. But researchers have been pulling feathers out of the bats' poop for years. And now, in a journal called the Public Library of Science, Popa-Lisseanu and her colleagues say blood and tissue tests show that birds are indeed a major food source for the giant noctule bat.

POPA: It was something that shocked all bat scientists.

NIELSEN: Popa-Lisseanu's team compared the chemical fingerprints that insects and birds left in the bat's body. Insects leave one kind of mark, while birds leave another. The researchers found that in the summer, the bats ate only insects. But during the spring and fall migrations, they ate lots and lots of birds.

Greater noctule bats are far too rare to pose a major threat to migratory songbirds. Even so, this recent paper is attracting plenty of attention. Peter Marra, a songbird expert with the Smithsonian Institution, says that's because the things these bats can do are just so incredibly cool.

PETER MARRA: That's exactly it. And it makes complete sense because there's just so much biomass moving through the air, it's not surprising that you've got a species like this taking advantage of it. And it's really neat that a bat is doing it.

NIELSEN: Unless you're the songbird.

MARRA: Or you are the ornithologist who likes the songbirds, sure.

NIELSEN: Popa-Lisseanu says her research team is continuing its studies. Working with electronic radars, the researchers now hope to catch one of the giant bats as it homes in on a flying songbird.

John Nielsen, NPR News, Washington.

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