Iron oxides in the plumage of bearded vultures. Medicine or cosmetics?

We have hypothesized (Negro et al. 1999) that bearded vultures, Gypaetus barbatus, deliberately stain their plumage with iron oxides to signal dominance status. The conceptual frame to which we ascribe our hypothesis is the handicap principle (Zahavi & Zahavi 1997). For a signal to convey reliable information, and thus to evolve, it has to be costly (Zahavi 1975), and we proposed that the associated cost of the red colour signal for bearded vultures is that adequate ferruginous springs are a limited resource.A.M. was supported by Department de Medi Ambient of the Autonomous Government of Catalonia. J.J.N. and F.H. acknowledge grant PB97–1264 from the Spanish Ministry of Education.Peer Reviewe