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Great Tits eat bats in times of need

September 25, 2009

Necessity is the mother of invention: Great Tits eat hibernating common pipistrelle bats under harsh conditions of snow cover. This remarkable newly-acquired behaviour was observed by researchers from the Max Planck Institute for Ornithology in Seewiesen and their colleagues in a cave in Hungary. When the researchers offered the birds alternative feed, they ate it and showed little or no interest in flying into the cave again. (Biology Letters, online prepublication from September 9, 2009)

Reports on the ingenuity of birds of the tit family in their search for food go as far back as the 1940s when it was observed that Blue Tits in the British Isles had learned how to open the aluminium tops of milk bottles left on doorsteps by milkmen to get at the cream that had formed on top of the milk. Another astonishing acquired behaviour among Great Tits (Parus major) has now been observed by the researchers working with Björn Siemers from the Max Planck Institute for Ornithology and their Hungarian colleagues. On 21 observation days over two winters, Great Tits flew a total of 18 times into a cave in north-east Hungary to look for and eat the Common Pipistrelle bats (Pipistrellus pipistrellus) hibernating there. The researchers explain this behaviour with the extreme necessity they faced in their search for food. Great Tits eat insects or arachnids in summer and usually look for seeds and berries in winter. Winters in north-east Hungary can be very harsh, however, with closed snow cover.

The bats give themselves away with their defensive sounds

The observed cave had a large entrance. Therefore, a small amount of light penetrated into it and the birds were able to find their way around in the semi-darkness. The birds probably locate the bats in the cave through the sounds that hibernating animals make when disturbed and awoken. These sounds range from the human acoustic range to the ultra-sound range. The researchers proved that these sounds were within the audible range of the birds by playing a recording of the bat sounds to them. The birds responded by moving towards the loudspeaker with interest. "These sounds probably have a defensive purpose," says Bjorn Siemers, "and it would seem plausible that the birds use the sounds to locate the bats." The Great Tits needed at most 15 minutes from entry into the cave to capture a bat. In some cases, they carried the Pipistrelles out of the cave in their beaks and ate them on nearby trees.

Flexible adaptation to the food supply

This behaviour is strongly dependent on the available food supply, however. When the researchers placed an additional food source in the form of sunflower seeds and bacon fat a few metres away from the entrance to the cave, only one of the birds still went looking for a bat. "Behavioural flexibility coupled with altered environmental conditions, e.g. food scarcity, can trigger astonishing innovations in animal behaviour," concludes Siemers. This innovative behaviour is not an isolated case and is probably passed on from generation to generation. Péter Estók, the first author of the study, had already observed a Great Tit eating a bat in the same cave ten years ago. A similar observation was also reported in Poland. "This could support the idea of cultural learning between different populations or it could indicate an independent development in different locations with the same ecological conditions," notes Björn Siemers in conclusion.

Max Planck Institute for Ornithology