Abstract
Both social and ecological factors influence population process and structure, with resultant consequences for phenotypic selection on individuals. Understanding the scale and relative contribution of these two factors is thus a central aim in evolutionary ecology. In this study, we develop a framework using null models to identify the social and spatial patterns that contribute to phenotypic structure in a wild population of songbirds. We used automated technologies to track 1053 individuals that formed 73 737 groups from which we inferred a social network. Our framework identified that both social and spatial drivers contributed to assortment in the network. In particular, groups had a more even sex ratio than expected and exhibited a consistent age structure that suggested local association preferences, such as preferential attachment or avoidance. By contrast, recent immigrants were spatially partitioned from locally born individuals, suggesting differential dispersal strategies by phenotype. Our results highlight how different scales of social decision-making, ranging from post-natal dispersal settlement to fission–fusion dynamics, can interact to drive phenotypic structure in animal populations.
Highlights
Group dynamics are an important part of an individual’s social landscape
We investigate the potential for social dynamics to contribute to evolutionary processes, as non-random association of phenotypes can lead to variable selection [33]
Morphometric measurements, including age and sex were recorded for every bird caught as adults (93% of all birds in this study). This marking protocol has been supplemented by intensive autumn and regular winter catching, in order to ring and tag immigrant birds, as part of a study into their social ecology. This approach enabled us to maintain in excess of 90% of the population fitted with passive integrated transponder (PIT) tags [23]
Summary
Group dynamics are an important part of an individual’s social landscape. Group size can impact predation risk through dilution [1], selfish herd dynamics [2] or predator confusion effects [3]. If the benefits of being in a group vary with the phenotypes of its members, we should expect regular patterns of associations to emerge between phenotypes in a way that maximizes the individual fitness of participants (such as kin structure in cooperative breeders [13]). How these patterns emerge in populations, whether from social (attraction or avoidance) or spatial (acceptance or exclusion) effects remains largely unexplored [14]
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