Scientists: burrowing owls use dung to "fish" for beetles
September 1, 2004
GAINESVILLE, Fla. — An old adage says you can catch more flies with honey than vinegar. But the burrowing owl has come up with another alternative: manure.
University of Florida scientists have a new explanation for burrowing owls’ odd habit of collecting and scattering animal feces within and around their shallow burrows: The owls are simply using the feces as bait to attract a favorite insect meal – dung beetles.
The findings are significant because they are based on what the scientists say are the first controlled experiments of tool use – dung as bait, in this case – by wild animals, a hot issue because of the enormous difficulty of interpreting animal behavior.
An article about the research will appear Thursday in the journal Nature.
“What makes this study unique is its experimental approach, documenting how effective tool use is. Tool use in general is a very controversial field because it’s often difficult to know whether an animal is doing what you think it’s doing,” said Doug Levey, a UF professor of zoology and the lead author of the paper. “Tool use has aroused a lot of controversy because of this problem of interpretation and because it’s fairly rare to see it. Consequently, most the reports are descriptive or anecdotal.”
Burrowing owls, known scientifically as Athene cunicularia, range from Canada to Chile, with a handful of small populations in Florida. Observers throughout their range have long noted their curious habit of hoarding cow, horse, buffalo, dog and other dung in and around the entrance to their nesting burrows, shallow holes that may reach 3 feet in depth and 6 to 9 feet in length.
There’s a general consensus that these birds use the dung as nesting material. But some scientists also have advanced another theory: The owls rely on the dung to mask the scent of their eggs from snakes, raccoons and other would-be predators.
Levey said a UF undergraduate ornithology class’ routine outing to view a burrowing owl population in North Florida led to the beetle hypothesis. He and others noticed that the indigestible pellets the owls had regurgitated contained large numbers of dung beetle parts.
That jibed with the owls’ other odd behavior. Unlike most other types of owls, burrowing owls are active during the day, when they often can be seen seemingly standing sentry outside their burrows. Many dung beetles also forage during the day, flying about 3 feet off the ground and sniffing the air for their next meals.
“You can go out there and see these owls standing in front of their burrows and it looks like they’re not doing anything,” Levey said. “But I think it’s pretty clear that they’ve got that old line in that water, fishing for these beetles.”
To test the hypothesis, the researchers removed all the dung from the ground surrounding about a dozen of the owls’ burrows at the sites of two separate North Florida populations. They then added similar amounts of cow manure to half the burrows, leaving the others without any. After four days, they collected all the pellets and prey remains near the burrows, then repeated the experiment by switching the control and experimental burrows, putting manure by those that had none in the prior test.
Examinations of the pellets and beetle parts around the burrows revealed that at those with dung, the owls “consumed 10 times more dung beetles and six times more beetle species than when dung was not present,” according to the Nature paper.
The researchers also tested the theory that the owls use the dung to mask their eggs’ scent. The researchers dug 50 artificial burrows and inserted five quail eggs into each, then scattered dung around half of the burrows. The researchers used commercially available quail eggs to avoid damaging the owl populations. Far from avoiding the eggs, predators quickly plundered all but one nest, showing that the dung made no difference to predators.
The researchers do not believe the owls evolved their behavior solely to attract dung beetles, Levey said. For one thing, the birds collect the most dung in the spring, largely abandoning the behavior in other seasons. For another, they collect a variety of other material. Among the oddest items the UF researchers discovered: squashed toads apparently peeled off nearby roads.
Levey also cautioned that the researchers make no claims the owls are consciously using the dung as bait, but rather that it has simply proved to be an effective and lasting behavior from an evolutionary standpoint.
“Even though the common perception of owls is that they’re wise, I don’t for an instant believe that these owls are aware of the connection between the dung that they bring and the beetles that they eat,” he said. “There’s no evidence of that. A simpler explanation is that those owls that bring back dung get more dung beetles, have higher reproductive success and pass that behavior on to their offspring.”
The research was funded with a $500 grant from UF’s University Scholars undergraduate research program. The paper’s other authors are R. Scott Duncan, a faculty member at Birmingham-Southern College in Alabama who earned his doctorate at UF, and Carrie Levins, who earned her bachelor’s degree in zoology at UF.