Perils of migration: new evidence that bats stalk birds.

Big Mediterranean bats snatch migrating songbirds out of the night sky in spring and fall, according to a new study.

When researchers proposed that idea in 2001, "there was so much controversy," says Ana Popa-Lisseanu of the Donana Biological Station in Seville, Spain. Now, she and her colleagues have cooperated with the idea's main critic, a conservation biologist at the University of Bern in Switzerland, to settle their argument.

The bats' blood chemistry points to bird feasts during migration season, the former disputants now agree. Their joint study appears online in the February PLoS ONE.

Billions of birds travel across the Mediterranean region twice each year. Most migrants use what had seemed to be safe flyways hundreds of meters above ground at night.

Yet danger now appears to loom, although no scientist has reported seeing a bat snag a bird. In 2000, two researchers in Italy reported that droppings from the giant noctule bat (Nyctalus lasiopterus) contained bits of bird feathers. Debate flared the next year after the Donana researchers also reported feathers in noctule droppings (SN: 8/11/01, p. 86).

In 2003, Bern conservation biologist Raphael Arlettaz and a colleague published a contrary scenario: Stray feathers waft down from migrating birds, and bats mistake them for night-flying insects. "Feathers in droppings are no proof that you eat bird flesh but certainly [are proof] that you swallow feathers," says Arlettaz.

After Popa-Lisseanu joined the Seville group in 2003, it began looking for a good test of whether bats eat birds. The team decided to track the bat's diet by measuring the ratios of rare-but-stable forms to the common forms of carbon and nitrogen in the bats and their potential prey. Working with Arlettaz, the researchers established that the migrating birds have higher ratios of carbon-13 and nitrogen-15 than the local insects do.

For 2 years, the researchers periodically took blood samples from as many giant noctules as they could catch. Monthly numbers varied, for example, from 3 to 18 during spring, summer, and fall of 2003.

The isotope ratios in a bat's blood change within a day to reflect what it's eaten. In summer, the researchers found, the ratios stayed relatively low, indicating a regular insect diet. In spring, the ratios were a bit higher, and in fall, they jumped out of the insect-diet range.

The results support the idea that bats prey on migrating birds. To get the observed rise in isotope ratio, the bats must actually be digesting bird tissue, the researchers conclude.

"I was one of the major detractors," says Arlettaz, "but with good evidence, I have now changed my mind completely. The most virulent detractors become the best proponents."

Michael J. Ryan of the University of Texas at Austin, who has studied bats that eat frogs, says that the new finding "makes perfect sense, as long as the bats are big and the birds are small."