We tested some costs and benefits associated with variable levels of mobbing response towards nest predators by American robins. Playbacks of robin mobbing calls attracted a major predator of robin nests, the northwestern crow. This demonstrates a potential cost to robins that give mobbing calls. We then used human 'predators' to test whether reproductive success was related to mobbing intensity. We first showed that mobbing responses to humans resembled those shown to a stuffed crow. Second, we demonstrated that responses of pairs of robins were consistent at different tests at the same nest, but were less consistent between different nesting attempts of the same pair. The first result validates our experimental procedure, but the second result suggests that variation in mobbing response is partly determined by characteristics of the nest or nest site, rather than by the level of aggressiveness of the parents. When we compared mobbing responses by robins at exposed and well-concealed nests, robins with exposed nests used extreme responses (swoops and hits) more frequently than those with concealed nests. We did not, however, find an consistent relationship between mobbing intensity, stage of the nestling cycle, or reproductive success. Robins did not respond more strongly late in the nesting cycle, and pairs that responded weakly, or strongly, experienced similar levels of nesting success.