WASHINGTON (Reuters) - Bats, lauded for scooping up mosquitoes and other nasty pests but reviled for drinking blood and spreading rabies, now have another unpopular habit to live down -- it appears they eat songbirds, scientists said on Tuesday.

Spectators in Austin, Texas watch bats fly at dusk in an undated file photo. Bats, lauded for scooping up mosquitoes and other nasty pests but reviled for drinking blood and spreading rabies, now have another unpopular habit to live down -- it appears they eat songbirds, scientists said on Tuesday. REUTERS/Pete Silva

Spanish and Swiss researchers said they had nailed down controversial evidence that one large species of bat preys on little birds as they migrate through the dark of night over the Mediterranean.

They said giant noctule bats, large bats with an 18-inch (45-centimetre) wingspan, were eating mostly insects during the spring but appeared to have a diet heavy in bird meat during the autumn.

No other animal preys on birds that migrate at night, and this species of bat may have switched to this abundant food source recently, they reported in the Public Library of Science journal PLoS ONE.

“In the course of a few million years, bats colonised most ecological niches and learnt to exploit a wide array of food sources including arthropods, pollen, fruit, small terrestrial vertebrates and even blood,” Ana Popa-Lisseanu and Carlos Ibanez of the Consejo Superior de Investigaciones Cientificas in Seville, Spain, and colleagues wrote.

Researchers on the team had earlier reported finding bird feathers in the faeces of the bats, creating a storm of controversy, with some biologists saying the bats must have accidentally eaten feathers floating in the air.

So the Swiss and Spanish researchers decided to look for more direct evidence. It is hard to tell what is going on in the middle of the night high in the air over the sea, so they analysed the blood of the bats.

Chemical variants called isotopes can tell what an animal has been eating and carbon and nitrogen isotopes are especially useful for pinpointing the sources of a diet.

The researchers tested the blood of the bats throughout the year and found strong evidence that the flying mammals ate only insects in the summer, ate a few songbirds in the spring, and then preyed heavily on birds in the autumn.

This could be because both parents and their naive young migrate from Africa back to Europe in the autumn, the researchers said.

“Every year, approximately five billion passerines (songbirds) cross the Mediterranean basin during their autumn migrations,” the Ibanez team wrote in their report.

“A big proportion of them are small-sized; as an example, more than 90 percent of migrating passerines mist-netted in the study area have an average body mass of less than 20 g (0.8 ounces). They thus represent a multitude of potential hunting targets for a large bat like Nyctalus lasiopterus.”

The ability of giant noctule bats to catch migrating birds while they fly at night appears to be unique, Ibanez and colleagues said.

“So far, no predator had been reported to exploit this extraordinarily diverse and abundant food reservoir represented by nocturnally migrating passerines,” they wrote.