Evolution of female coloration: What have we learned from birds in general and blue tits in particular

Abstract Female ornaments have long been considered non-functional, but a paradigm shift has occurred over the 2 last decades. The adaptive nature of female ornaments is now widely accepted. After a rapid overview of this shift, we present the results of comparative studies focused on identifying the forces involved in the evolution of female coloration in birds. We then discuss the results of intraspecific ornithological field studies and finish up by summarizing the work done by our group and others on female coloration in blue tits (Cyanistes caeruleus). Overall, this review confirms that female coloration traits function as ornaments and/or badges of status in many bird species. It also identifies several mechanisms that can circumvent trade-offs in investment between coloration traits and egg production. Based on this review, we call for further research on certain topics and specific changes in practices. More precisely, at the macroevolutionary level, we should avoid framing our questions around sexual dichromatism and male-centered proxies of sexual selection if we wish to elucidate the female-specific selective forces and constraints involved in the evolution of female coloration. At the microevolutionary level, we need to quantify social and sexual selection in both sexes, and to perform experimental studies to compare the selective forces acting on female and male coloration. In particular, it appears important to investigate how maternal effects and physiological drivers of aggressiveness relate to female coloration. Finally, our work on blue tits illustrates the importance of conducting long-term studies in tandem with replicated experiments within a given species.