This bird distributes animal dung in and around its burrow to provide a bait for its prey.
Reports of tool usage by birds tend to be anecdotal as only a few individuals may be involved1 and the behaviour observed can often be interpreted in other ways2,3. Here we describe the widespread collection of mammalian dung by burrowing owls (Athene cunicularia) and show that they use this dung as a bait to attract dung beetles, a major item of prey. Our controlled investigation provides an unambiguous estimate of the importance of tool use in a wild animal4.
Burrowing owls place mammalian dung in and around their burrows5 (Fig. 1). If removed, it may be rapidly replaced, suggesting that it is more than an incidental accumulation of debris6. Because burrowing owls are one of the main predators of dung beetles5,7 and stand motionless for long periods by their burrows, we investigated whether they could be using mammalian dung as bait to ‘fish’ for dung beetles. The largest common species of dung beetle at our study site (Phanaeus igneus; 2 cm in length) is diurnal, as are the owls. This species makes up 65% of the beetles consumed by the owls (n=20 owl pellets analysed). Alternatively, mammalian dung may mask the scent of the nest from predators6, given that the nests of burrowing owl are accessible to a wide range of terrestrial predators and that many nests are lost to them5.
To test this idea of olfactory camouflage, we created 50 nest burrows, spaced 50 m apart, and placed five quail eggs in each; every alternate burrow received cow dung. We recorded nest fate every two days for 3.5 weeks, which is a typical incubation period. All but one nest was discovered and destroyed by predators. Survival analysis revealed no difference in time to destruction between nests with and without dung (Cox log-hazard ratio of 0.62±0.31, z=1.58, P>0.10). We conclude that dung does not effectively mask the scent of eggs. It may, however, be effective at masking the scent of chicks, which we did not test for ethical reasons.
To test the baiting hypothesis, we first removed all dung, regurgitated pellets and beetle parts from the burrow entrances of two owl populations. Half received 231±16 g (dry mass) of cow dung, typical of the amount found at a burrow entrance; the remainder received no dung but were otherwise treated in the same way. After four days, we collected all prey remains and regurgitated pellets from the burrows and repeated the experiment, this time switching the treatment and control burrows. We found that when dung was present at the burrows, owls consumed ten times more dung beetles and six times more dung-beetle species than when dung was not present (P<0.001; Fig. 2).
A few instances of tool use by wild birds are wonderfully detailed and widely accepted1,4. More generally, however, the ecological and evolutionary significance of tool use is difficult to judge because evidence for how such behaviour benefits wild animals is scant4. Trials with captive animals have shown that they have surprising abilities8, but such results are difficult to extrapolate to natural settings. Baiting with dung and then waiting for dung beetles to come is akin to the use by herons of floating objects (such as bread, feathers and insects) as tools to attract fish9, although the foraging success with and without ‘bait’ has not been compared. Our experiment provides such a comparison, demonstrating that tool use can substantially benefit a wild animal.
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The authors declare no competing financial interests.
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Levey, D., Duncan, R. & Levins, C. Use of dung as a tool by burrowing owls. Nature 431, 39 (2004). https://doi.org/10.1038/431039a
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