Carotenoid-based plumage colouration is predicted by age and parasites in the male European serin

A fundamental assumption of theories on the evolution of sexual signals is that they should be costly to produce in order to honestly signal the quality of the sender. The expression of carotenoid-based plumage signals is considered to be condition-dependent, due to the role of carotenoids functioning as pigments and as health modulators. We assessed carotenoid-based plumage colouration in relation to male condition in a free living population of male European serins Serinus serinus during the breeding season. Male serins were trapped for morphometric and colouration measurements, during a four-year field study, in order to evaluate the signalling value of colouration in relation to body condition and parasites level. We compared two different forms of colour quantification based on spectral data - the most commonly used tristimulus colour variables and physiological models of avian colour vision - and found that they were highly correlated for this species. We investigated the signalling value of male plumage colouration and it was found to be related to age and ectoparasite load. Plumage double cone and patch size were negatively related to parasites level, whereas SWS ratio was positively related to parasites and age. Colouration was also related with the time since moult. Our results indicate that the colour expression of serin's plumage is age dependent and is related, in complex ways, with the ability to cope with parasitic infection.