A Hungarian cave has turned out to be a larder of highly unusual food for great tits: hibernating pipistrelle bats.
Over two winters, researchers found the birds were systematically hunting bats by sight and sound as they hibernated through the cold months.
Writing in the journal Biology Letters, the scientists say this is the first proof of bat-hunting in songbirds.
Great tits usually dine on smaller prey such as insects and seeds, with bat-eating probably very rare.
The researchers found the birds preferred other food when they could get it.
"It doesn't look like this is an overwhelming thing that threatens the bat population," said Bjorn Siemers from the Max-Planck-Institute for Ornithology in Seewiesen, Germany, one of the research team.
"So then the question to ask is 'how do they invent it?', and so far we can only speculate - it could be a kind of cultural learning," he told BBC News.
In previous decades, researchers have reported finding dead or injured bats outside caves in Sweden and Poland.
In one case, the bat was being eaten by a great tit, in others they bore wounds that could have been inflicted by a tit's beak; but there was no proof that the birds had hunted or killed them.
If they did, said Dr Siemers, it raised the possibility that the habit or skill of bat-hunting was carried along migration routes.
During two field seasons in the Bukk Mountains of northeastern Hungary, the researchers documented 16 cases of great tits (Parus major) hunting, killing and eating a hibernating bat in the one cave.
Pipistrelle bats (Pipistrellus pipistrellus) are about one-quarter of a great tit's size.
The birds would fly close to the cave walls, landing frequently and often disappearing into crevices. They would either eat the bats there and then or carry them away for feeding.
When their hibernation is disturbed, the bats squeak in the audible range for humans and great tits.
The researchers speculated that the birds may have learned to listen for these squeaks - and when they recorded some and played them back, the birds responded with interest about 80% of the time.
However, the team believes the birds can only bat-hunt when they can see their prey, as they can in this wide-mouthed cave where lots of light penetrates - again suggesting it would be a rarely-found behaviour.
In a later experiment, the scientists provided other food - sunflower seeds and bits of bacon - and found that the birds preferred to eat those, leaving the bats largely untouched. This raises the suggestion, said Dr Siemers, that bats are a food of last resort in a harsh winter.
Graham Madge, a spokesman on conservation issues for the UK's Royal Society for the Protection of Birds (RSPB), said it was new to him.
"Normally, great tits are feeding on things like insects, beetles, spiders, seeds and maybe fruit in winter - there's no indication they'd be able to predate something like a bat. It's incredible behaviour."
He noted that in the UK, a close relative, the blue tit, has also been quick to take advantage of novel foods.
"There was this phenomenon where blue tits learned how to open the foil tops of milk bottles, and quickly this behaviour spread through the population; so they're quick learners," he said.