Acoustic signalling in Eurasian penduline tits remiz pendulinus: Repertoire size signals male nest defence

Elaborate male song may restrain competitors in various songbirds, although the exact mechanism, information content and information flow of acoustic signals are not completely understood. Here we focus on the interactions between resident and intruder males using the Eurasian penduline tit Remiz pendulinus. The breeding system of this small passerine bird is unusually variable including sequential polygamy by both sexes, and appears to be driven by intense sexual selection and sexual conflict over parental care. We had two objectives in this study: (i) to investigate whether male repertoire size is an intra-sexual signal in penduline tits, and (ii) to determine emitter and receiver roles during acoustic communication between residents and intruders. We tested these objectives in a natural population by recording the resident males' song and then challenging them by song playbacks. The residents' approach to intruder stimuli and behavioural responses were monitored. We found that intruder repertoire size did not evoke different responses by the residents. Rather, the resident's approach distance and behavioural response was predicted by his own repertoire size, suggesting that song in penduline tits is involved in male-male communication, and repertoire size may function as a reliable signal of the resident male's ability and willingness to defend his nest and avert possible intruders