"Melanins are the most common pigments accounting for the color of animals. Melanic traits often evolve as reliable signals of genotypic quality. It has been proposed that this process depends on the levels of a key intracellular antioxidant (i.e. glutathione; GSH), which are low during the production of one of the main types of melanin (i.e. eumelanin), but high during the production of the other form (i.e. pheomelanin). Therefore, the current model for the evolution of melanic traits gives a key role to oxidative stress, but it has not been properly tested yet. The project proposed here aims at testing this evolutionary scenario. First, an experiment will be performed with captive house sparrows Passer domesticus in order to determine if an experimental reduction of GSH levels leads to a higher expression of a reliable eumelanic signal and a mobilization of alternative antioxidant resources only in birds of high genotypic quality. The second objective will be to determine how the availability of an amino acid with antioxidant activity that serves as precursor for a substrate for pheomelanin (i.e. methionine) and that of an amino acid without antioxidant activity (i.e. phenylalanine) that serves as precursor for a substrate of melanins, interact to affect the expression of a eumelanic signal in house sparrows under different conditions of exogenous oxidative stress. The third objective will be to determine how radioactive contamination affects the expression of, and role of sexual selection in reliable eumelanic and pheomelanic signals of great tits Parus major and barn swallows Hirundo rustica breeding in Chernobyl through decreases in GSH and thus increases in oxidative stress levels. Finally, an investigation in a breeding colony of barn swallows of known ages will be performed in order to test the hypothesis that the reliability of pheomelanic signals increases with age because of an accumulation of free radical effects and thus a decrease in GSH levels."