Female song occurs in songbirds with more elaborate female colouration and reduced sexual dichromatism (#276)
Elaborate plumages and songs in male birds are classic evidence for Darwinian sexual selection. Critically though, trait elaboration in birds is not gender-restricted: female song has recently been revealed as a taxonomically-widespread trait within Passeriformes, prompting a surge of research into likely functions and socioecological correlates. Here we perform the first test for an evolutionary association between female song and plumage colour elaboration (male, female, and sexual dichromatism) in songbirds, using phylogenetically-informed comparative analysis. We use published data on female song for ~600 species of songbirds and a novel approach that allows for the reliable and objective comparison of colour elaboration between species and genders. On the one hand, a negative correlation between acoustic and visual elaboration is expected if there is an evolutionary trade-off between signalling modes. This hypothesis (termed the ‘transfer’ hypothesis) has been commonly proposed in males but has mixed empirical support. On the other hand, a positive correlation between female song and female plumage elaboration is expected if these traits have similar or overlapping functions and evolve under similar selection pressures. Our results reveal a significant positive correlation between female colourfulness and female song presence: females who sing tend to be more colourful then those who do not. However, males are not more colourful in species where females sing, reducing the degree of sexual dichromatism in these species. These results suggest that female plumage and female song may have evolved together under similar selection pressures and that their respective functions are reinforcing. We discuss the potential roles of sexual versus social selection in driving this relationship, and the implications for future research on female signals.