Sex differences in bird ornamentation: female plasticity, ecological and physiological constraints

Sexual dimorphism in bird ornamentation is well-documented, with male traits often emphasized in sexual selection studies, while female ornamentation remains understudied. Investigating European serins (Serinus serinus) and common waxbills (Estrilda astrild), I summarize experimental evidence revealing that sex differences in ornamentation arise from sex-specific ecological and physiological constraints, particularly affecting females. In European serins, female ornamentation does not function as a sexual or dominance signal, contrasting with males. In common waxbills, female bill colouration is highly plastic and sensitive to ecological conditions and oestradiol levels, leading to context-dependent sexual dichromatism that can be reduced or eliminated under favourable conditions. Together, these results challenge the view that sexual dichromatism is predominantly genetically fixed and male driven, highlighting female colouration plasticity as a key source of variation in ornamentation and social signalling. These findings underscore the need to integrate female colour variation for understanding the evolution of sexual and social signals.