Multimodal duetting in magpie-larks: how do vocal and visual components contribute to a cooperative signal's function?

Many animals communicate using multimodal signals, which simultaneously address different senses. Components may prompt similar or different responses when used alone, whereas the composite signal may have a similar meaning to one or both components, or a novel meaning. We used robot models to dissect the roles of visual and vocal components in the multimodal duets of Australian magpie-larks, Grallina cyanoleuca. Mated pairs produce antiphonal vocal duets, and coordinate them with visual duets, which usually entail alternating wing movements. Previous playback experiments show that purely vocal duets have a territorial function, but the role of visual displays is unknown. We measured territorial responses by pairs to each component independently, as well as to the multimodal signal, and analysed the choice of song and visual display used by partners during duets. Acoustic and multimodal playbacks prompted the same, strong territorial response, entailing both flight and vocal duetting by the resident pair. By contrast, purely visual playbacks did not prompt a strong territorial response. Furthermore, pairs used the same visual display within multimodal displays, independent of song type, implying that birds watch their partners when choosing the visual display, and suggesting that it could be a within-pair signal. We conclude that the vocal component of duets is a territorial signal to other pairs, whereas the visual component might either coordinate vocal duets within pairs, analogous to a conductor's baton, or have a restricted role in communication between rival pairs, perhaps especially at long range.