Hungry great tits hunt for hibernating bats

When food is precious, animals can resort to strange behaviours in order to satisfy their hunger. Take the great tit. Its usual diet of insects and creepy-crawlies is harder to come by in winter. But in one Hungarian cave, great tits, ever the opportunists, have learned to exploit a rich and unusual source of food. They kill sleeping bats.

i-8bff6ddbd9fd55cd6de2946e7feb7fd3-Pipistrelle.jpgGreat tits are only about 5 inches long, but their prey - the pipistrelle bat - is smaller still, just an inch or two in size. The bats spend the winter months hibernating in rock crevices. They're well hidden, but when they wake up, they start making noises and these are the telltale signs that the birds are listening out for. They hunt by flying slowly and systematically across the cave walls, eavesdropping on the bats' noises, and killing them while they're still woozy.

Peter Estok from Germany's Max-Planck Institute for Ornithology spent two winters watching a group of around 50 great tits hunting for bats. Previously, there had only been a smattering of anecdotal evidence that this happened. In one case, a tit was found eating a dead bat outside a Polish cave, but it could well have been scavenging off an already deceased corpse. Then, thirteen years ago, Estok saw a great tit capturing a live bat in a Hungarian cave. He was intrigued and he returned to the cave several times for more observations.

Tits lack the obvious killing apparatus of birds of prey but their short beaks are strong enough to dismember a tiny pipistrelle. Estok saw several instances of actual kills and recovered a few carcasses that showed obvious bite wounds. The bodies were picked clean enough to suggest that the birds were killing the bats for food and not, say, to remove competition for roosting spots. 

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These attacks are driven by necessity. On days when Estok hung a feeder outside the cave entrance, provisioned with seeds and bacon, he saw only one instance of a tit killing a bat. Without the feeder, he saw 17 such incidents.

Estok even confirmed the tits' hunting technique by recording the noises of waking bats and playing them back from a speaker hidden in the rocks. Around 80% of the birds reacted strongly to the sounds, approaching the speaker and investigating more closely. This is especially interesting because other studies have found that the calls of waking bats actually put off mammal predators - they tell them that the bats are awake and not susceptible to ambushes. But for birds, which can rival the bats in the air, these calls are far from a deterrent. 

The odds of a great tit surviving for more than 8 years are one in a thousand. This means that the individual that Estok saw eating a bat in 1996 couldn't possibly be part of the same group that he studied this time round. Has the bat-killing behaviour passed down through generations of tits as a local cultural tradition? It's impossible to say for now, but there's certainly precedence for this - British blue tits famously learned to open milk bottles to drink the cream at the top and the behaviour spread like wildfire across the country.

Reference: Biology Letters doi:10.1098/rsbl.2009.0611

Image: Great tit by Luc Viatour; bat from paper.

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By Nathan Myers (not verified) on 09 Sep 2009 #permalink

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