DISSERTATION RESEARCH: Aggressive Signaling and Fitness Consequences in a New World Warbler

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Disputes often arise among animals competing for limited resources. In such circumstances, animals may avoid direct combat through use of stereotyped communication signals, such as visual or vocal displays. Prior studies by the co-PI on vocal signaling in a songbird, the black-throated blue warbler, showed that two vocal display features are particularly "honest" signals of threat, in terms of forecasting future aggression. Specifically, the production of extremely quiet songs ("soft" songs) and a certain song category ("type II" songs) both reliably predict whether a bird will subsequently attack a model opponent. The first part of proposed research will test whether these two signal features convey differing levels of threat, and if and how birds may escalate aggressive interactions by modulating these two song features either alone or in sequence. Towards this end, a field experiment will simulate interactive song contests that escalate in intensity. Birds that respond with type II songs during low levels of escalation are predicted to give soft songs during higher stages of escalation, with each higher stage becoming a more reliable predictor of eventual attack. This experiment will indicate whether animal disputes follow a hierarchical "ladder" of escalation, as has been predicted in recent theoretical work. The second part of proposed research asks if and how animals benefit through their use of threat displays. Natural variation among males in threat display use will be documented, and "winners" and "losers" of vocal interactions coded based on who backs down first. Genetic paternity analysis will provide estimates of correlations between threat displays and reproductive success. Males that more frequently use threat displays and "win" more interactions against opponents are predicted to be more successful at siring offspring. This project will provide field research experience for undergraduates, and will be showcased in outreach activities to elementary school students.