Is host activity necessary to elicit brood parasitism by cowbirds?

Abstract

Brood parasites must first find their host's nests before they can parasitize them, yet little has been reported about the nest searching behavior of brood parasites, even for the well-studied Molothrus cowbirds. A few studies have attempted to identify particular searching cues used by cowbirds for discovering nests, but all have been unable to discriminate among two alternative hypotheses: the necessity of adult hosts as cues for nest-finding (the Host Activity Hypothesis) versus the discovery of nests by search of habitat without need for presence of an adult host (the Habitat Search Hypothesis). In this study, we developed a field experiment that improved our ability to discriminate among these hypotheses. We varied the visual conspicuousness of real, but inactive, passerine nests that we placed in situations typical of the particular species involved and supplied the nests with fresh passerine eggs. We predicted that if adult hosts were necessary to elicit brood parasitism, no experimental nests would be parasitized. Further, if cowbirds find nests by searching habitat without regard for host presence, we predicted that conspicuous nests would be parasitized more frequently than inconspicuous nests. Despite the extraordinarily high levels of cowbird parasitism on natural nests in the study region, none of the experimental nests was parasitized. Thus, host activity appeared to be a critical cue for eliciting parasitism. Although our results are consistent with the Host Activity Hypothesis, discriminating between nest discovery with subsequent parasitism and nest discovery without subsequent parasitism remains problematic. We argue that the presence of the adult host at or near a nest is essential at some stage of the parasitism event, either for initial discovery of the nest or for timing the laying of the parasitic egg in a manner that maximizes its likelihood of successfully producing offspring.