Adaptive social behavior frequently involves discriminating between classes of individuals such as relatives versus non-relatives, older versus younger individuals, or individuals of different status. In the absence of spatial cues, this discrimination may be based on signals that correlate with fitness-related traits (e.g., older or high-status males may sing higher performance songs) or with identity, for example, when receivers distinguish and classify signalers based on their unique signal structure. Here, we examine vocal age-based discrimination in western bluebirds (Sialia mexicana), a North American songbird in which older males have a significant advantage in obtaining extra-pair fertilizations, and therefore pose a significantly higher threat to paternity than younger males. We asked whether western bluebird males showed a higher response to playback of songs of older males compared to younger males relative to their own age. We prepared song stimuli by removing three potential signals of age that have been identified as important in other species: (1) note consistency (which was achieved by playing a single instance of each note repeatedly), (2) note repertoire size, and (3) singing rate (the latter two were equalized across conditions). Even in the absence of these potential signals of age, young males responded more strongly to playback of older males’ songs than to young males’ songs, suggesting that they are able to discriminate between age classes relative to the threat they pose. Further research is required to determine whether this discrimination is based on individual recognition or signal features that are correlated with age.
Extra-pair mating Individual recognition Dear enemy effect Communication networks Western bluebirds
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This research was supported by funding from the National Science Foundation (NSF IOS-0097027 and NSF IOS-0718416 to J.L.D). We thank a large number of field volunteers who over the year helped to collect a large share of the data analyzed in this paper. We also thank Hastings Reserve (Vince Voegeli and Jaime Del Valle) for logistical support for the field work and two anonymous reviewers for helpful comments on the manuscript.
Compliance with ethical standards
All procedures and protocols were designed in accordance with the ASAB/ABS Guidelines for the use of Animals in Research and were approved by the Institutional Animal Care and Use Committee, Cornell University (protocol number #2005-0137). All research was conducted with the appropriate federal and state banding permits.
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