Impact of visual contact on vocal interaction dynamics of pair-bonded birds

Animal social interactions usually revolve around several sensory modalities. For birds, these are primarily visual and acoustic. However, some habitat specificities or long distances may temporarily hinder or limit visual information transmission making acoustic transmission a central channel of communication even during complex social behaviours. Here we investigated the impact of visual limitation on the vocal dynamics between zebra finch, Taeniopygia guttata, partners. Pairs were acoustically recorded during a separation and reunion protocol with gradually decreasing distance without visual contact. Without visual contact, pairs displayed more correlated vocal exchanges than with visual contact. We also analysed the turn-taking sequences of individuals' vocalizations during an exchange with or without visual contact. In the absence of visual contact, the identity of a vocalizing individual was well predicted by the knowledge of the identity of the previous vocalizer. This property is characteristic of a stochastic process called a Markov chain and we found that turn-taking sequences of birds deprived of visual contact were Markovian. Thus, both the temporal correlation between the calls of the two partners and Markov properties of acoustic interactions indicate that, in the absence of visual cues, the decision to call is taken on a very short-term basis and solely on acoustic information (both temporal and identity of caller). Strikingly, when individuals were in visual contact both these features of their acoustic social interactions disappeared indicating that birds adapted their calling dynamics to cope with limited visual cues.