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Owls Use Dung to "Fish" for Beetles
Burrowing owls have an affinity for the dung of other animals. Their underground nests and surrounding areas are carpeted with the stinky stuff. Now a team of researchers has found at least one reason why all this fecal matter matters to the owls: It's bait for dung beetles, the owls' favorite grub.
The research, reported in today's issue of the science journal Nature, demonstrates that burrowing owls (Athene cunicularia) deliberately use mammal dung as a tool to reel in a meal—and in the process substantially increase the number of dung beetles they eat.
"Burrowing owls are diurnal [active in daytime], they will sit at the burrow entrance all day long and it looks like they're doing nothing," said Douglas Levey, a zoologist at the University of Florida in Gainesville. "But you know what? It's pretty clear they are fishing. They've got their line in the water and are waiting for dung beetles to come by."
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Baiting and Waiting
Levey, who is the lead author of the study, added that the finding is particularly noteworthy because it demonstrates that tool use can substantially benefit a wild animal. Such convincing evidence is scant in the biological record.
For example, herons are widely known to place a floating object, such as a bread crumb or feather, on top of water as a lure for minnows. But no studies have compared how well herons would do if they did not fish in this manner.
"As far as I know, [our study] is the first example of that, not just in birds but in wild animals in general," Levey said.
Gavin Hunt, an ecologist at the University of Auckland in New Zealand who studies tool use by crows, agrees.
"The owl paper is the first to really quantify the benefit of tool use to a free-living animal in terms of food gained," he said. "As such, it is an interesting finding that seems to confirm what most of us have only assumed to date."
The researchers came up with the study idea several years ago while on a field trip to observe burrowing owls. It was part of an ornithology course Levey was teaching at the University of the Florida.
Levey and his students—including study co-authors Scot Duncan, now at Birmingham-Southern College in Alabama, and Carrie Levins, who lives in Panama City, Florida—noticed dung scattered around the burrow and wondered why it was there.
Duncan, whose wife happens to be a beetle expert, recognized beetle parts in the regurgitated pellets that were mixed in with the dung. He realized that the owls were eating a bunch of beetles.
"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.
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 material—owls, dung, and dung beetles—if 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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