Use of social over personal information enhances nest defense against avian brood parasitism

Interactions with conspecifics influence the behavioral repertoire of an organism, as they apply to foraging techniques, song acquisition, habitat selection, and mate choice. Few workers have studied the role of social interactions in molding defense responses, especially the defense of the nest. We tested the effect of social interaction on nest defense of the reed warbler (Acrocephalus scirpaceus), one of the main hosts of the brood-parasitic common cuckoo (Cuculus canorus) in Europe. This parasite reduces its host's breeding success; therefore, any response that prevents successful parasitism should be selected. Because of their high nesting density and consistent cuckoo-specific responses that also recruit conspecifics, reed warblers may benefit by acquiring already-competent antiparasite responses from conspecifics instead of incurring the costs implicit in trial-and-error attempts. Using treatments that included presentations of taxidermic mounts, clutch manipulations, and playbacks, we tested the effect of conspecific defense on the response intensity of nesting reed warblers. Exposure to social cues resulted in an increase of cuckoo-specific nest defense responses, whereas experience with natural or experimental parasitism did not produce any change in defense intensity. The preferential use of social cues rather than personal experience as it applies to the enhancement of mobbing was consistent with what was found in other behaviors, where the strategy adopted by individuals was the result of costly acquisition of personal information and reliability of social cues.