Pair-Coordinated Calling: Eurasian Magpies Respond Differently to Simulated Intruder Pairs That Overlap or Alternate Their Calls

Animal vocalisations are widely used to signal strength or motivation of a caller in competitive interactions, such as in territorial defence. Substantial understanding of signalling functions in territorial conflicts is based on singing by male songbirds. Yet, in many species, both pair members call during territorial conflicts, as well as in predator-induced situations, leading to complex signalling interactions in which calls overlap or alternate. This raises the question as to whether or not variation in how individuals in pairs time their calls is perceived as meaningful by receivers. Here, we tested with playback experiments whether Eurasian magpies (Pica pica), a species producing alarm calls (so-called chatter calls) in territorial defence, respond stronger to simulated pair-intruders who overlap their calls with each other than to those who alternate them. Magpies emitted a significantly longer first chatter calls in response to playback with overlapping calls but chattered significantly sooner and approached the loudspeakers significantly more closely in response to playbacks of alternating (and therefore longer) call sequences. These findings exemplify that the timing of calls by pair members matters, but in more complex ways than we predicted. The overlapping playback appeared to trigger a longer yet later initial chatter response and a weaker approach response, suggesting that the different ways in which magpies respond reflect different levels of arousal or defence strategies. The results may also reflect uncertainty by receivers due to a potential mismatch between signalled and perceived information: While overlapping calls may signal high arousal by both callers, a longer alternating sequence could be perceived as a more aroused longer signal. These findings expand on classical experiments on call function, suggesting that pairs can vary the message by coordinating their alarm calls in different ways, similar to how duetting pairs time their song contributions in advertisement signalling.