SOCIAL CONTEXT AND THE EVOLUTION OF DELAYED REPRODUCTION IN BIRDS

Classic life history theory makes generalized predictions about phenotypic correlations across large clades. Modern comparative tests of these correlations account for the underlying structure of phylogenetic trees. Yet neither life history theory nor phylogenetic comparative methods automatically specify how biological mechanisms generate correlations. This problem is evident in comparative analyses of birds. Birds show a correlation between body size and age at first reproduction, but do not actually grow larger if they delay reproduction. Instead, field studies raise the hypothesis that social contexts—especially cooperative breeding, coloniality, and lekking—generate unique demands for behavioral development, which in turn result in delayed reproduction. Here, we support that hypothesis with a comparative dataset spanning 961 species in 155 avian families. Continuous (Ornstein-Uhlenbeck), discrete (hidden state Markov), and phylogenetic regression models revealed delayed reproduction in colonial birds, a weaker signal in cooperative birds, and the consistent evolution of sexual bimaturism in polygynous, lekking birds. These results show an association between diverse social contexts, sex-specific developmental demands, and life history evolution in birds. Considering this diversity, we discuss how even statistically powerful phylogenetic correlations—whether focused on mass, lifespan, or broad social categories—can ultimately fail to model the history of life history evolution.