Wednesday, September 09, 2009

Killer birds bite off bats heads

Killer birds bite off bats' heads

It sounds like the avian equivalent of an Ozzy Osbourne legend. Great tits have been discovered killing and eating bats by pecking their heads open.

Although bats have been reported preying on songbirds before, this is the first time great tits have been observed to prey on bats.

Péter Estók of the Max-Planck-Institute for Ornithology, Germany, first saw a bat being captured by a tit in a Hungarian cave in 1996.

Ten years later, he and fellow bat ecologist Björn Siemers recorded 18 examples of pipistrelle bat predation by great tits, over the course of two winters in the same cave in the Bükk Mountains.

Bat hunting

The birds seek out bats as they wake from hibernation and usually eat them in the cave, though sometimes they carry them to a nearby tree.

"The birds don't kill the bats before they start eating them," says Siemers, "but the bats eventually die when the birds peck open their brain case."

As the bats are still very cold, only a degree above ambient temperature, they are extremely slow and easy for the birds to subdue. Nevertheless, it is a considerable feat for the tits given that a pipistrelle weighs approximately 5 grams and a great tit only four times as much.

Sly birds

The scientists ran an experiment to supply the tits with food, and discovered that this reduced consumption.

"This shows that the birds are predating the bats for food in times of scarcity. It shows how inventive this species can be," says Siemers.

Estók and Siemers also ran a playback experiment where they recorded and played the calls that the bats made as they woke from hibernation. The great tits were attracted to the calls.

"For a bat, these calls are very low frequency, a maximum of 15 kiloHertz, but at a high frequency for the tits, above scientifically established hearing levels for this species, yet they react to them," says Siemers.

Who started it?

Gareth Jones, an expert on bat behaviour at the University of Bristol, says the finding is unexpected and novel. "I don't know of any other studies of predation of hibernating bats by small birds. It's a big jump for the tits, given that their normal prey are caterpillars."

As the birds have been anecdotally observed to eat bats in this cave for a decade, Siemers speculates that this is an example of cultural transmission. There are also four anecdotal reports of bats being eaten by tits in Sweden and Poland.

Jones says, "Presumably this is learned behaviour, but it is much too strong an inference to suggest that it could be culturally transmitted from Poland to Hungary."

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2009.0611 (in press)