Extra-pair paternity drivers plumage colour elaboration in male passerines

The elaborate ornamental plumage displayed by birds has largely been attributed to sexual selection, whereby the greater success of ornamented males in attaining mates drives a rapid elaboration of those ornaments. Indeed, plumage elaboration tends to be greatest in species with a high variance in reproductive success such as polygynous mating systems. Even among socially monogamous species, many males are extremely colourful. In their now-classic study, Moller and Birkhead (1994) suggested that increased variance in reproductive success afforded by extra-pair paternity should intensify sexual selection pressure and thus an elaboration of male plumage and sexual dichromatism, but the relatively few measures of extra-pair paternity at the time prevented a rigorous test of this hypothesis. In the nearly three decades since that paper's publication, hundreds of studies have been published on rates of extra-pair paternity and more objective measures of plumage colouration have been developed, allowing for a large-scale comparative test of Moller and Birkhead's (1994) hypothesis. Using an analysis of 186 socially monogamous passerine species with estimates of extra-pair paternity, our phylogenetically controlled analysis confirms Moller and Birkhead's (1994) early work, demonstrating that rates of extra-pair paternity are positively associated with male, but not female, colouration and with the extent of sexual dichromatism. Plumage evolution is complex and multifaceted, driven by phylogenetic, ecological, and social factors, but our analysis confirms a key role of extra-pair mate choice in driving the evolution of ornamental traits.

Automated summary

The elaborate ornamental plumage displayed by birds is largely attributed to sexual selection, with males' success in attracting mates driving the rapid evolution of such traits. This study examines the hypothesis that increased variance in reproductive success resulting from extra-pair paternity intensifies sexual selection pressure, leading to the elaboration of male plumage and sexual dichromatism. Through a large-scale comparative analysis of socially monogamous passerine species, the findings confirm the hypothesis and demonstrate a positive association between extra-pair paternity rates and male plumage coloration and sexual dichromatism. This study highlights the significant role of extra-pair mate choice in the evolution of ornamental traits.