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 in response to playback with overlapping call but tended to more closely approach the loudspeakers playing back 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 stronger initial chatter response but a weaker approach response, suggesting that the different ways in which magpies respond reflect different levels of arousal or defence strategies. 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.

Methods
We conducted playback experiments simulating a territorial intrusion by a pair of magpies were the timing of calls from the pair members differed. Specifically, we used two loudspeakers broadcasting chatter-calls with the calls from the second speaker either alternating or overlapping the calls from the other loudspeaker. The playback trials were conducted in the vicinity of magpie nests. A trial consisted of one minute playback plus two additional minutes observation time. During the trial, we recorded magpie vocal responses and noted the closest approach distance to the loudspeakers of any magpie. Audio recordings were analysed in Audacity® v3.1.3 (Audacity Team, 2023) by setting labels over all chatter calls and overlapping chatter sequences. Those labels were exported as text files and merged with trial data in R v4.3.2 (R Core Team, 2023). We used the packages tidyr v1.2.0 (Wickham & Girlich 2022) and dplyr v1.0.8 (Wickham et al. 2022) to extract for each trial a) the closest approach distance of magpies to the speakers, b) the total number all chatter calls (including those that overlap with another) and total duration of chattering (i.e. total duration of all single chatter calls and overlapping chatter sequences), and c) the number of overlapping chatter sequences. For those trials in which chattering occurred, we further extracted d) the latency between the start of the playback and the first chatter and e) the number of syllables of the first chatter. This dataset contains the resulting summary file per trial. The details of the methods are described in the manuscript itself