In British gardens they are welcome visitors, stopping off to sing and snack on the caterpillars. But in a remote cave in Hungary they are exhibiting some unusual behaviour. There, great tits have turned into predators who search out and eat roosting bats.

Scarcity of normal dietary staples is believed to have led the birds to prey on the creatures, the first time the extraordinary behaviour has been seen.

A team of scientists, led by Peter Estók at the Max Planck Institute for Ornithology in Germany, reported that on hearing the pipistrelle bats squeak the great tits entered the cave in north-east Hungary and started to peck at them as they sheltered in crevices. Whether the birds are an isolated group who have learned to prey on these bats in the cave, or whether great tits in Europe are altering their eating habits in a similar way, is uncertain, the scientists say.

The researchers staked out the cave and spent 22 days observing over two winters. On hearing the bat calls the birds flew slowly along the roof of the cliff where the animals nestled. The birds were recorded killing bats 16 times.

"We found several carcasses and one was in what I would call a terminal state," said Estók. "A great tit had pecked away all of the muscle, fat and intestines and even the brain. All that was left was skin and bones. This is the first time these birds have been seen capturing and eating bats." The researchers found that when offered sunflower seeds and bacon scraps the tits stopped flying into the cave. A report on the behaviour appears today in the Royal Society journal Biology Letters.

The cave is well served by natural light, so the tits can see to home in on their prey. To investigate whether they came across the bats by accident or were truly hunting them, the scientists broadcast bat sounds from speakers close to the mouth of the cave. On hearing the calls almost all of the great tits flew up to the speaker to take a look.

The researchers speculate that the great tits may be taking advantage of food available in their immediate environment – as blue tits in Britain have done, opening doorstep milk bottles to drink the cream.