Identifying timescales of change in vulture social networks

Animal social interaction patterns change over time, but the continuous nature of social interactions makes selecting a timescale for temporal analysis challenging. We applied both a heuristic approach and a multilayer reducibility analysis approach to study timescales of change in social networks of free-ranging griffon vultures. We analyzed social networks in two behavioral situations: in-flight interactions, such as during foraging movements, which we expected to fluctuate seasonally but to exhibit a relatively constant pattern of change over the course of a season; and diurnal ground interactions, such as interactions while feeding, which we expected to show a pulsed temporal pattern that followed the pattern of carcass availability on the landscape. The heuristic method confirmed the suitability of a 3-10 day aggregation window for studying temporal change in vulture social networks. It also highlighted how different timescales of aggregation offer different insights about longer-term patterns of change. Multilayer reducibility analysis confirmed that substantial change was happening at every aggregation timescale we tested, with no redundancy in network layers; that is, social interactions in this population were not oversampled. However, it revealed more similarity between non-adjacent layers in the flight networks as compared to the feeding networks, further supporting the influence of carcasses as drivers of social network structure. Multilayer reducibility analysis over a multi-season timescale did not reveal seasonal similarities in network structure extensive enough to override the substantial demographic and tag coverage turnover between seasons. While multilayer analysis of temporal dynamics in social networks may prove useful for the study of change in links among a fixed subset of individuals, we highlight its limitations as a tool for studying long-term social network structural change, especially in free-living animal populations.