AbstractKleptoparasitism (one organism stealing prey from another) is especially common in birds. Avian kleptoparasites should be especially likely to target insects such as digger wasps (Hymenoptera: Sphecidae) that occur in large aggregations and repeatedly deliver large prey to the same nesting site. We document kleptoparasitism of digger wasps (Sphex pensylvanicus L.) by sparrows (Passer domesticus (L.)) and catbirds (Dumetella carolinensis (L.)). During summer 2008, we observed 697 wasp provisioning attempts (return of a prey-laden wasp to a marked nesting site) in a mown field. One-third (244/697) of attempts were intercepted by birds, primarily sparrows, which increased their hourly total number and hourly number of successful attacks per capita. Wasps maintained a consistent rate of successful provisioning attempts but may not have been able to do so indefinitely. Energetic costs related to kleptoparasitism may exert strong selection pressures on affected digger wasps.