Kleptoparasitism, an interaction in which one organism steals another organism's already-acquired prey/resources, occurs in a range of taxa and ecosystems. It is especially common in birds, a group that includes both opportunistic and specialist kleptoparasites. Opportunistic avian kleptoparasites should be especially likely to target insects such as digger wasps, large hymenopterans that provision underground nests with paralyzed prey to feed their developing young, occur in large aggregations, and return to the same nest over several days or weeks. There is, however, little evidence for such bird-wasp interactions in the published literature.

We report the results of research documenting kleptoparasitic interactions between great digger wasps (Sphex pensylvanicus), House Sparrows (Passer domesticus), and Gray Catbirds (Dumetella carolinensis). During summer 2008, we observed bird-wasp interactions in a mown field in Foster RI (USA).

Results/Conclusions

We observed 697 wasp provisioning attempts (defined as the return of a prey-laden wasp to a marked nesting site). One-third (244/697) of these attempts were intercepted by birds that harassed the wasp into dropping the prey for the bird to consume. Although the number of successful wasp provisioning attempts/hr remained stable across the sampling period, the total number of provisioning attempts/hr increased. This is explained by the fact that the percentage of wasp provisioning attempts harassed by birds increased from 42% to 93% over the observational period, leading to a 40% decrease in the percentage of successful wasp provisioning attempts.

House sparrows were the primary kleptoparasites, and an increasing number of them congregated at the wasp's nesting sites and remained there throughout the observation period. The sparrows became more aggressive towards the wasps over time, and increased their per capita number of total attacks/hr and successful attacks/hr. They did not, however, become more efficient kleptoparasites; the percentage of sparrow attacks that succeeded in obtaining prey did not change over time.

Although wasps apparently maintained a consistent number of successful provisioning attempts over the course of the observational period, it is unlikely that they could have continued to do so indefinitely. This may provide one explanation for the fact that bird-wasp kleptoparasitism appears rare and restricted to specific locations. Such 'natural experiments' may be especially useful in light of the apparently high costs of the kleptoparasitism, and provide a forum for addressing the question of how (and whether) digger wasps compensate for such dramatic reductions in provisioning success.