Mechanisms of colour-production modulate the link between extra-pair paternity and avian sexual dichromatism

Most birds are socially monogamous, but extra-pair paternity (EPP) is widespread. EPP can increase variance in male reproductive success and the opportunity for sexual selection, potentially leading to sexual dichromatism. While the link between EPP and sexual dichromatism is well established, over two decades ago, Owens and Hartley suggested that this relationship is mainly driven by structural colours, that is, colours that are created by the interaction between light and the feather microstructure, presumably because of their role in active sexual displays. However, this conclusion was based on a small sample of species. Here, we expand these analyses investigating the relationship between EPP and male and female colouration produced by three colour-producing mechanisms-melanin-based, carotenoid-based, and structural colouration-across over 400 bird species. Our results reveal that EPP relates to colouration across all three mechanisms. In species with high EPP levels, males tend to have more black and less light brown, whereas females are often more cryptically coloured, with less red and blue. Species with high EPP levels also exhibited more colour-producing mechanisms. Our study suggests that EPP can enhance sexual dichromatism by favouring conspicuous male and cryptic female colouration.