Since the dawn of abstract thinking, humans have wondered about the seemingly unnecessary elaborate ornamentations of birds. Gaudy colours, cumbersome tails, complex vocalizations and bizarre displays are found in bird species from all corners of the globe. Darwin (1871) provided an elegant explanation for the existence of these non-utilitarian traits: they increase mating success, and although they may impair survival, the costs of producing and bearing elaborate ornaments can be repaid in the currency of additional offspring. Darwin’s model still serves as the foundation for our concept of sexual selection but great strides have been made in our understanding of sexual selection processes since his time (e.g. Zahavi 1975, 1977, Lande 1980, Hamilton & Zuk 1982, Kirkpatrick 1982, Grafen 1990, Andersson 1994). The great majority of work to date has focused on species in which males alone are elaborately ornamented. Far less has been published on the function of ornaments that are expressed in both sexes (Kraaijeveld et al. 2007), a condition sometimes termed ‘elaborate monomorphism.’ As such, the question remains whether the strong generalizations that we make regarding male ornamentation also apply to species in which both sexes are ornamented.
Tarvin, K. A., & Murphy, T. G. (2012). It isn't always sexy when both are bright and shiny: considering alternatives to sexual selection in elaborate monomorphic species. Ibis, 154(3), 439-443. doi:10.1111/j.1474-919X.2012.01251.x