Are two heads better than one? Responses of the duetting kokako to one- and two-speaker playback

Duets may serve a variety of functions, including the defence of both territories and pair bonds. In species with antiphonal duets, single birds could theoretically perform songs structurally identical to those of pairs. How, then, could receivers detect whether song comes from a pair or an unpaired bird, and what would be the competitive advantage of duetting if two birds were not necessary to produce full song? We investigated whether an antiphonally duetting species, the kokako, Callaeas cinereus wilsoni (previously Callaeas cinerea wilsoni) distinguishes pairs from unpaired birds by detecting two spatially separated sources of sound. We predicted that simulated duets would represent greater territorial threats than simulated solo songs, and would therefore elicit stronger aggressive responses. We presented kokako with playback of themes sung entirely by one singer, with (1) all theme phrases broadcast from one speaker and (2) theme phrases split (in a way natural for kokako duets) between two speakers placed 10 m apart. Kokako responded differently to one- and two-speaker playbacks. Pairs perceived the simulated duet as the greater territorial threat, as shown by their faster approaches to two-speaker playback. Pairs also matched (responded to a playback phrase by singing the same phrase type) and anticipated (sang a phrase type immediately before it was broadcast by the playback) more often during two-speaker playbacks. We examined pair proximity during playbacks to determine whether simulated duets were perceived as a stronger threat to the pair bond than solo song, and found no support for this second hypothesis.