Vocal kin recognition in kin neighborhoods of western bluebirds

In most cooperatively breeding birds, individuals direct helping behavior to close relatives. Western bluebirds live in family groups in winter and show a high degree of male philopatry. Sons disperse locally forming kin neighborhoods and occasionally help at their parents' or brothers' nests. Although the pattern of kin-directed helping is well established in birds, the mechanism of recognition is known in only a few cases. Here, we report on an experiment with western bluebirds (Sialia mexicana) that investigates the role of vocalizations in kin recognition, a species that even when breeding on its own often maintains lifelong connections with kin. We presented western bluebirds with songs recorded from equidistant kin and nonkin living on other territories, conducting playback near their nests on 2 consecutive days. We found that male western bluebirds responded more aggressively to playback of nonkin song compared with kin song. These results suggest that vocal signatures serve as a kin recognition cue in western bluebirds. We discuss these results within the context of other systems in which vocal kin recognition has been demonstrated and discuss the potential for further research to examine signal development and to distinguish kin signatures from individual signatures, which can also be used to recognize kin.