COLOURATION AS AN INTRASPECIFIC SIGNAL IN BIRDS

Colour variation seems to be a pivotal trait in avian communication dynamics, where information is provided to recipients by ‘signallers’ through colouration. Plumage colours can be structural or due to pigments incorporated into the keratin of feathers, and melanin-based colouration, produced by the melanocortin system, is one of the most common in birds. Melanin-based colouration is under genetic control and the degree of melanism is often associated with variation in physiological, behavioural and life-history traits, due to pleiotropic effects of the compounds regulating the melanin synthesis. However, variability in colouration is observed not only among plumage traits but also in fleshy ornaments, legs or eggshells. Variation in eggshell pigmentation, determined by biliverdin and protoporphyrin pigments, may accomplish several different functions, the most obvious of which are camouflage and background matching. It has been also proposed that intra-specific variation in eggshell pigmentation patterns could reflect egg, maternal or paternal traits and hence provides reliable cues to conspecifics about egg, maternal or paternal quality. The first part of the studies presented in this thesis is aimed at investigating the role of melanin-based colouration from different points of view. I started by investigating the role of melanin-based colouration in barn swallow nestling (Hirundo rustica) as a signal of individual quality and its covariation with behavioural stress response. Moreover, I investigated the covariation of genes regulating melanin-based plumage colouration and plumage polymorphism in lesser kestrel males (Falco naumanni). Secondly, since the relationship between protoporphyrin-based eggshell pigmentation and egg or maternal/paternal traits appears to be highly variable among species, I investigated the possible signalling role of protoporphyrin-based eggshell pigmentation in both study species. Using both experimental and correlative approaches, I found that even if pleiotropic effects of melanocortin system may still drive the associations between melanin-based colouration and individual quality, the melanocortin pleiotropic hypothesis may not apply to plumage colouration deriving from mixing of eu- and pheomelanins. Moreover, results on the potential signalling role of protoporphyrin-based eggshell pigmentation in two distantly related bird species support the idea that intraspecific signalling via eggshell pigmentation is species-specific feature rather than a general pattern among avian taxa.