Is host activity necessary to elicit brood parasitism by cowbirds?

Brood parasites must first find their host's nests before they can parasiti ze them, yet little has been reported about the nest searching behavior of brood parasites, even for the well-studied Molothrus cowbirds. A few studie s have attempted to identify particular searching cues used by cowbirds for discovering nests, but all have been unable to discriminate among two alte rnative hypotheses: the necessity of adult hosts as cues for nest-finding ( the Host Activity Hypothesis) versus the discovery of nests by search of ha bitat without need for presence of an adult host (the Habitat Search Hypoth esis. In this study, we developed a field experiment that improved our abil ity to discriminate among these hypotheses. We varied the Visual conspicuou sness of real, but inactive, passerine nests that we placed in situations t ypical 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 c owbirds find nests by searching habitat without regard for host presence, w e predicted that conspicuous nests would be parasitized more frequently tha n inconspicuous nests. Despite the extraordinarily high levels of cowbird p arasitism on natural nests in the study region, none of the experimental ne sts was parasitized. Thus, host activity appeared to be a critical rue for eliciting parasitism. Although our results are consistent with the Host Act ivity Hypothesis, discriminating between nest discovery with subsequent par asitism and nest discovery without subsequent parasitism remains problemati c. We. argue that the presence of the adult host at or near a nest is essen tial 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 ma ximizes its likelihood of successfully producing offspring.