Physiological costs and age constraints of a sexual ornament: an experimental study in a wild bird

Sexual ornaments are often considered honest signals of quality because potential costs or constraints prevent their display by low-quality individuals. Testing for potential physiological costs of ornaments is difficult, as this requires experimentally forcing individuals to produce and display elaborate ornaments. We use this approach to test whether a sexually selected trait is physiologically costly to male superb fairy-wrens (Malurus cyaneus). Male fairy-wrens moult from brown to blue breeding plumage at different times of the year, and females strongly prefer the few males that are blue early, during winter. We used short-acting testosterone implants to stimulate males to produce ‘early-blue’ plumage and assessed costs during and after moult using a panel of physiological indices. Testosterone-implanted, T-males moulted in winter and produced blue plumage six weeks before control-implanted, C-males. T-males moulted while in lower body condition, had lower fat reserves, and were mo...