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.

Figure 1
figure 1

R. G. Wolff

Baiting and waiting: an owl stands at the entrance to its burrow, surrounded by the dung that it has positioned to entice beetles.

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).

Figure 2: Number of dung beetles consumed by burrowing owls in the presence (blue) and absence (orange) of cow dung positioned at their burrows.
figure 2

Averages and standard errors are shown; n=10 burrows per treatment in two populations (Gilchrist County, Florida). P<0.001 for both the number of individuals and the number of species (in paired t-tests). Beetles were identified by their elytra, which owls commonly discard before consumption or regurgitate in pellets. Because we restricted our sampling of prey remains to the immediate vicinity of each burrow (to an area of about 0.75 m2), the total number of beetles consumed during the trials will have been higher than indicated.

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.