Summary
Aggression to model brown-headed cowbirds measured at nests of 34 potential host species shows a positive correlation with rate of parasitism. The data are used to test two hypotheses: (1) that aggression functions as an antiparasite defense reducing parasitism, and (2) that cowbirds use host aggression as a cue when searching for nests.Our results suggest that host aggression only serves as an effective antiparasite defense in dense host populations. In low-density populations aggression is probably used as a nest-finding cue by cowbirds, because there are too few neighbouring hosts to mount an effective multispecies mobbing response. Although being aggressive is maladaptive in the latter situation, the evolution of increased aggressiveness is evident, because individuals breeding in dense populations have greater fitness. The adaptive complex is further complicated because host aggression may have a major innate component and a minor learned component.Experiments conducted on different host populations corroborate the hypothesis that hosts with the longer history of sympatry with cowbirds are more aggressive. These data are consistent with the predictions of a simple directional selection model for host aggression.