Social environment and the evolution of delayed reproduction in birds

One puzzling feature of avian life histories is that individuals in many different lineages delay reproduction for several years after they finish growing. Intraspecific field studies suggest that various complex social environments—such as cooperative breeding groups, nesting colonies, and display leks—result in delayed reproduction because they require forms of sociosexual development that extend beyond physical maturation. Here, we formally propose this hypothesis and use a full suite of phylogenetic comparative methods to test it, analyzing the evolution of age at first reproduction (AFR) in females and males across 963 species of birds. Phylogenetic regressions support increased AFR in colonial females and males, cooperatively breeding males, and lekking males. Continuous Ornstein-Uhlenbeck models support distinct evolutionary regimes with increased AFR for all of the cooperative, colonial, and lekking lineages. Discrete hidden state Markov models suggest a net increase in delayed reproduction for social lineages, even when accounting for hidden state heterogeneity and the potential reverse influence of AFR on sociality. Our results support the hypothesis that the evolution of social environments reshapes the dynamics of life history evolution in birds. Comparative analyses of even the most broadly generalizable characters, such as AFR, must reckon with unique, heterogeneous, historical events in the evolution of individual lineages.