By Peter Wood
BBC News Online
Owls have been observed using dung as bait to trap beetles for food, the journal Nature has reported.
Baiting and waiting: The owls may entice beetles with dung
Scientists have likened the behaviour to "tool use", something that is rarely seen amongst wild animals.
US experts compared what owls ate when there was a typical litter of dung at the entrances to their nests with their diet when the dung was removed.
The owls ate 10 times more beetles when the dung was present, suggesting the waste did not build up by accident.
Douglas Levey, from the University of Florida, and his colleagues studied two colonies of burrowing owl, for whom dung beetles are a favourite prey.
As the name suggests, burrowing owls (Athene cunicularia) make their nests in small tunnels. Outside the entrance, they collect a variety of debris, including dung.
Levey says that "pretty much every burrow contains dung" but "if you remove the dung, at least in the spring they will replace it".
He added this suggested the dung was more than an incidental accumulation of debris.
"Maybe there's something about the dung itself. If you remove pieces of carpet or aluminium foil they won't replace it," Professor Levey told BBC News Online.
Learning that burrowing owls also have a high concentration of dung beetles in their diet, the scientists proposed that the owls might be using dung as bait to attract the beetles.
Scientists describe such behaviours, where animals use intermediate methods to achieve an objective - such as baiting to acquire food - as "tool usage".
Taking the bait
Researchers constructed their experiment by clearing all the nest entrances of debris, then one owl colony had a typical littering of dung applied whilst the other was left bare. After four days each entrance was again completely cleared and the situation was reversed.
Analysis of the owls' waste clearly showed that when dung was present, the owls feasted on 10 times more dung beetles. As Levey says, "this experiment demonstrates that tool use makes a difference to a wild animal".
Although it may be tempting to conclude the owls are clever enough to devise this trap, Levey explained: "I don't believe these burrowing animals are aware of the link between the dung they bring in and the beetles they catch".
Instead, the baiting may simply have evolved, as owls who happened to collect more dung had a better diet and therefore bred more successfully.
It is widely known that captive animals practice tool usage, particularly when trained. But Professor Levey notes that until now "reports of tool usage by birds tend to be anecdotal and the behaviour observed can often be interpreted in other ways".