They are a favourite with British birdwatchers, but great tits have a dark side, if they don't have enough food they and become meat-eating killers.

German researchers observed how, when times are tough, the tits will hunt, kill and peck the meat off bats for food.  

The team from the Max-Planck Institute say that over two winters they saw how the birds targeted bats hibernating in caves.

"Over two winters, we directly observed 18 predation events. The tits specifically and systematically searched for and killed bats for food," said one of the researchers … who should have just put some nuts out.