"Everybody who studies burrowing owls knows they bring dung back to their burrows, and they know that burrowing owls eat a lot of dung beetles. But nobody had put two and two together," Levey said.
To test the hypothesis that the burrowing owls were indeed luring the dung beetles to their nests with dung, the researchers removed dung and pellets from several burrows. They then placed fresh dung at the entrance to some and left others bare.
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Four days later the researchers examined the owls' regurgitated pellets and found that the owls with dung bait ate ten times more dung beetles and six times more dung beetle species than the owls without dung bait.
But before you go out and buy your favorite burrowing owl a kitschy "gone fishin'" sign to hang on the burrow wall, Levey said there is no evidence to suggest the owls are actually aware of what they are doing.
"Even though people think owls are wise, there's no reason to assume they make a conscious choice to go get the dung they bring back because [they know] beetles will then appear out of nowhere," he said.
Rather, according to Levey, this dung beetle baiting behavior is likely a trait that evolved via natural selection: Owls that bring back more dung are more likely to get more dung beetles and thus are more likely to be successful in reproduction, passing on the trait.
"The owls are using a rather simple method to catch beetles with readily available material, so the raw materialowls, dung, and dung beetlesif you like, was just waiting for evolution to come up with tool use," Hunt, the New Zealand ecologist, said.
In fact, Levey doubts that the behavior of bringing dung back to the burrow evolved for the reason of dung beetle baiting. Owls bring other stuff back to the burrows, including bits of plastic, carpet tailings, foil, and gum wrappers, all of which may serve as insulation. Or, the dung may serve to camouflage from predators the scent of eggs or chicks.
The researchers tested the egg hypothesis using quail eggs and found that nests with and without dung were equally attacked by predators. However, the test did not rule out the possibility that chicks give off a smell that the dung masks.
"We didn't test that. We didn't want to sacrifice chicks, so the olfactory hypothesis may yet hold," Levey said.
Regardless, the researchers' finding that the dung beetle baiting behavior benefits the owls' diet is generally held to be valid.
"I'm convinced that owls with dung outside their burrows catch more dung beetles than when the dung is not there," Hunt said. "That doesn't rule out another advantage of having the dung around the burrows, though."
Levey said, "It's often the case that one behavior can do more than one thing, and there's nothing wrong with that."
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