Team of rivals: alliance formation in territorial songbirds is predicted by vocal signal structure.

Cooperation and conflict are regarded as diametric extremes of animal social behaviour, yet the two may intersect under rare circumstances. We here report that territorial competitors in a common North American songbird species, the chipping sparrow (Spizella passerina), sometimes form temporary coalitions in the presence of simulated territorial intruders. Moreover, analysis of birds' vocal mating signals (songs) reveals that coalitions occur nearly exclusively under specific triadic relationships, in which vocal performances of allies and simulated intruders exceed those of residents. Our results provide the first evidence that animals like chipping sparrows rely on precise assessments of mating signal features, as well as relative comparisons of signal properties among multiple animals in communication networks, when deciding when and with whom to form temporary alliances against a backdrop of competition and rivalry.