When under strong ecological pressure, or when a good opportunity arises, animals have often shown themselves to be surprisingly innovative in how they adapt to new pressures or take advantage of new resources. Many examples of this have been observed in the wild including the discovery of tool use by chimpanzees, problem solving in guppies and the development of a novel ‘body-slapping’ behaviour as a means of communication in grey seals. No behaviour has surprised me more however than the discovery that in Hungary a population of a small seed-eating song bird, the great tit (Parus major), has switched from its staple diet of seeds and insects and has learnt to search for, kill and eat hibernating bats (Pipistrellus pipistrellus).
At around five inches long great tits are small birds, but pipistrelle bats are even smaller at just an inch in size. During the winter these bats hibernate in cracks and crevices in dark caves or old buildings where they are safe and well hidden, but when they awaken they start making noises which draws the attention of nearby predators, including great tits.
The earliest suggestion that great tits might hunt for bats goes back to at least 1947 when a Swedish biologist named Olaf Ryberg observed dead bats in Sweden with “injury, caused e.g. by titmice (possibly also bigger birds)“. It was to be almost half a century before the subject was raised again when in 1996 a great tit was seen feeding on a dead bat in a cave in Poland. Three years later at the same site in Poland three more bats were found, one dead and two alive, with injuries which looked like they were caused by tit beaks. Despite these observations it was still not clear that in any of these cases great tits were actually hunting for bats actively and it remained a possibility that they were simply scavenging on bats which had already died. A chance observation of a great tit capturing a live pipistrelle in a cave in Hungary in 1996 provided the only evidence at this point that great tits ever actively preyed on live bats.
That first observation was made by Péter Estók from Germany’s Max Planck Institute for Ornithology and intrigued by what he had seen he and his research team returned to the cave in Hungary on three separate occasions from 2004 to 2009. Using experiments and old-fashioned observation they aimed to discover whether feeding on bats by great tits was simply opportunistic, or whether great tits had learnt to deliberately and systematically hunt for and feed on pipistrelles.
The research team quickly found their answer. During the first winter of observations they witnessed great tits capture and consume live bats seventeen times in just ten days. Yet despite this it was still not known why this behaviour had developed in the first place.
One possibility was that great tits used bats as a last-ditch food source when their regular food was in short supply. To test this possibility the researchers left a mixture of sunflower seeds and bacon in feeders around the cave entrance to provide an easy and irresistible meal for any passing great tits. Sure enough, when plentiful food was provided they found that hunting for bats by great tits stopped almost completely with only one case observed over a ten-day period. This provided good evidence that feeding on bats was driven by an urgent need for food and did not represent a more general shift in diet.
Now just one question remained to be answered. How do great tits find the bats in the first place? It was thought that they might be able to home in on the bat’s calls so to test this possibility Estók recorded the bats and played their calls back to great tits from a speaker. Around 80% of the birds reacted strongly to the sounds often turning their heads towards the speaker and approaching to investigate. This was particularly interesting because in one study bat calls were shown to act as a deterrent to mammalian predators, possibly by signalling that the bats are awake and cannot be caught. For great tits however it seems that bat calls are far from a deterrent, possibly because they can easily outmanoeuvre a bat in flight.
Eight years passed between the first observation of a great tit preying on a live bat and the start of Estók’s study. Given that the typical lifespan of great tits is three years the birds observed in 2004 couldn’t possibly have been the same birds that were seen in 1996. This raises the fascinating possibility that the bat killing behaviour is passed from one generation to the next by some form of cultural transmission. Whether this is or is not the case is not yet known and so it seems there is still much to learn about the unassuming great tit.
For the study of great tits hunting bats
Estók P, Zsebok S, & Siemers BM (2010). Great tits search for, capture, kill and eat hibernating bats. Biology letters, 6 (1), 59-62 PMID: 19740892
Bat calls as a deterrent to mammalian predators
Martin, K., & Fenton, M. (1978). A possible defensive function for calls given by bats (Myotis lucifugus) arousing from torpor
Canadian Journal of Zoology, 56 (6), 1430-1432 DOI: 10.1139/z78-196
Innovative behaviour in other animals
Body slapping seals
Bishop, A., Lidstone-Scott, R., Pomeroy, P., & Twiss, S. (2013). Body slap: An innovative aggressive display by breeding male gray seals (Halichoerus grypus) Marine Mammal Science DOI: 10.1111/mms.12059
Problem solving guppies
Laland KN, & Reader SM (1999). Foraging innovation in the guppy. Animal behaviour, 57 (2), 331-340 PMID: 10049472
Tool use in chimpanzees
Goodall, J. (1964). Tool-Using and Aimed Throwing in a Community of Free-Living Chimpanzees Nature, 201 (4926), 1264-1266 DOI: 10.1038/2011264a0