Social complexity does not lead to more stable demographic histories across bird species

Louis Bliard , Jiaqi Xu, Michael Griesser, Yiqing Chen, Damien R. Farine, Guojie Zhang, Shaohong Feng, Anna Brüniche-Olsen, David Nogués-Bravo, Ryan R. Germain

Species’ responses to environmental variability can include evolutionary changes in social behaviours, leading to variation among species in characteristics including their mating systems, care systems (e.g., cooperative breeding), the rate of group formation, and the strength of interaction between individuals. These social traits can influence the physiology of individuals, potentially shielding them from adverse environmental conditions, and thus ultimately affect the demography of populations and species. However, the demographic consequences of sociality have rarely been studied and have been limited to ecological timescales. Across evolutionary timescales (thousands-to-millions of years), anecdotal comparative evidence based on a few species suggests that social traits lower effective population size while also buffering species demographic responses to environmental change. Here, we used whole-genome sequence data to reconstruct the deep-time demographic history of 630 populations of 322 bird species over half a million years. We then explored the role of different components of social complexity traits (social organisation, mating system, care system) in buffering the long-term demography of species to broad-scale environmental change. Contrary to previous studies, our analyses found no clear evidence of social complexity influencing and stabilising the long-term demographic patterns of bird populations. This finding highlights that the role of sociality on species’ demography across evolutionary timescales might not be as widespread as previously assumed and instead be taxa-specific.