Evidence for the evolution of male and female cardinals’ plumage colouration being affected by both natural and sexual selection

The evolution of sexually selected traits, such as colouration, has been the object of considerable research. Among birds, plumage colouration is highly variable and includes some of the most exuberant signals. While sexual selection is generally considered the primary driver of the most conspicuous forms of these signals, several ecological, morphological, and social factors have also been shown to influence their evolution. Thus, a complex interaction of factors can drive the evolution of colouration in different groups of birds, which needs to be considered for a broader perspective. Cardinals constitute a monophyletic group of birds with an extensive palette of colour variation, including some species with quite exuberant colouration, covering a wide geographical and environmental range, making them an exciting group to study the possible main drivers of colour evolution. We considered factors related to natural and sexual selection to test their role in explaining cardinals’ plumage colour evolution. We also considered males and females separately, as they are likely subject to different selective pressures. We found that the evolution of male and female colour diversity and brightness was positively associated with the intensity of sexual selection. In females, it was also associated with colour saturation. Female colour evolution was also influenced by natural selection, including habitat cover (negatively related to colour diversity, saturation, and brilliance) and net primary productivity (negatively related to brilliance). Both sexes shared similar types of colouration sources, but diverged in colour expression. For carotenoid colouration, yellow is predominant in females, while males display reds more often, which agrees with the indicator hypotheses for sexually selected signals. These findings provide new insights into the evolution of cardinal colours and support the importance of both sexual and natural selection in shaping colour evolution. They also highlight the importance of considering the evolution of both male and female plumage colouration in the context of colour signalling evolution.