Eavesdropping on conspecific alarm calls links birds across territory borders into a population-wide information network

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Animal communication, including the transmission of risk-related information, has long been recognized as occurring in a network, rather than being limited to signaller–receiver dyadic interactions. However, it is rare for studies on communication to explicitly incorporate a network approach and for network studies to focus on communication interactions and information transmission in nongrouping species. Consequently, the properties of communication networks and the extent to which risk-related information can spread through populations of territorial animals remain largely unexplored. Here, I examined the structure and spatial extent of eavesdropping networks by studying alarm calls in wood warblers, Phylloscopus sibilatrix, small, territorial songbirds inhabiting temperate European forests. Using a playback experiment, I first established the active space of wood warblers' alarm signals, that is, how the strength of antipredator responses to conspecific alarm calls varies as a function of distance between signallers and receivers. Next, I combined social network analysis and fine-scale, long-term data on wood warblers' territory distribution to delineate spatially explicit alarm call eavesdropping networks and to explore the potential for risk-related information transmission at the population level. Response strength of wood warblers to conspecific calls decreased with increasing distance to callers, suggesting distance-dependent eavesdropping. However, each year the majority of individuals occupied territories within the active space of alarm signals of multiple neighbours and could reach information from even more of them via indirect links. Moreover, the patterns of connections between individuals were generally similar across years, suggesting that basic network properties remain stable over time. This study shows that during the spring breeding season, individual birds are interlinked with conspecifics across territory borders into population-wide information webs, in which risk-related information can readily spread across large spatial scales. In turn, this highlights the social environment as an important dimension of individuals’ niches even in otherwise territorial animals.