Mixed-species associations can arise without heterospecific attraction

Despite widespread research on the interaction rules that drive group-living behavior in animals, little is known about the spatial self-organization of individuals in heterospecific groups. This has led to significant challenges in teasing apart the various mechanisms thought to underpin multispecies groups. One potentially useful approach for gaining an understanding of this process is to identify the rules that best predict the observed distribution of individuals across these groups. In order to gain an insight into the decision-making process that might generate patterns of heterospecific associations, we collected data on the number and distribution of nests in breeding colonies that contained 3 species of weaverbird. We found no evidence of segregation by species, either within or between colonies. Using agent-based simulations of males applying different rules of attraction and repulsion to conspecifics or heterospecifics, we found that the best-fitting rule contained no heterospecific attraction. In this rule, individuals picked colonies based on an optimal distribution of conspecific nests. Given that nests are an important sexual signal in weavers, our findings suggest that this rule is biologically relevant: Males are seeking an optimal trade-off between attracting females via lekking and competing for mates if too many conspecific nests are present.