MELANIN-BASED COLOURATION AS A SIGNAL OF INDIVIDUAL QUALITY AND ITS POTENTIAL ROLE IN SEXUAL SELECTION IN THE BARN SWALLOW (HIRUNDO RUSTICA)

Bird colours can be structural or can be due to a wide range of pigments incorporated into the feather keratin such as porphyrins, carotenoids and melanins. Melanins are not acquired through diet but are synthesized within dedicated organelles called melanosomes. Despite the strong genetic control of melanin synthesis, environmental variables and physiological condition can influence the expression of melanin ornaments, suggesting that the degree of melanism may honestly signal individual quality and have a role in sexual selection processes. However, studies on the association between melanin-based colour traits and life-history strategies have often provided inconsistent results. Thus, identifying any association between melanisation and fitness traits in populations under a sexual and natural selection regime is pivotal to any study of the evolution of melanism and, more generally, to research on sexual selection . The studies presented in this thesis aimed at investigating the role of melanin-based colouration in the Barn swallow (Hirundo rustica) as a signal of individual quality both in nestlings and adults and its role in sexual selection processes. Moreover, the results of a meta-analysis on the intensity of sexual selection on different phenotypic traits across the six Barn swallow subpopulations are presented. By correlative and experimental approaches, this thesis provides novel findings about the role of plumage colouration as a signal of individual quality. Indeed, I demonstrated the parental ability to differentially invest in male and female offspring according to a phenotypic melanin-based trait expressed during early development which will be involved in intersexual competition at sexual maturation; in addition, I provided unprecedented evidence that ventral plumage colouration, involved in parent-offspring communication, reflects telomere length and thus offspring quality. Moreover I demonstrated that, in adult Barn swallows, ventral plumage colouration is a signal of telomere length and predicts seasonal, but not lifetime, reproductive success. Ventral plumage colouration was also found to predict promiscuity in female Barn swallow, suggesting that it may affect male choice of social as well as of potential extra-pair mates. Finally, the present thesis provides a quantitative meta-analytic support for the evidences that different ornaments are subject to different selection regimes among geographically distinct Barn swallow populations, according to the hypothesis that sexual selection plays a major role in speciation processes.