Does anthropogenic metal pollution affect carotenoid colouration, antioxidative capacity and physiological condition of great tits (Parus major)?

Although several studies have shown that carotenoid-based signals are negatively affected by (metal) pollution, the underlying mechanisms are not well understood. There are two possible, not mutually exclusive, hypotheses to explain the metal-induced fading of carotenoid colouration. Metal pollution could affect oxidative stress levels and/or the diet. We assessed the expression of the yellow breast of adult and nestling great tits (Parus major) and related this to physiological parameters in plasma indicative of oxidative stress (total antioxidative capacity) and nutritional condition (albumin, triglyceride, total protein, cholesterol and uric acid concentrations). In four study sites along a metal pollution gradient, both adult and nestling great tits had significantly reduced carotenoid colouration at the most polluted sites. While nestlings' total antioxidative capacity was significantly affected by metal pollution, there was no significant effect on adults' total antioxidative capacity. Both for adult and nestling birds, no clear relation between total antioxidative capacity and carotenoid colouration was found. However, there were significant differences among sites in nutritional parameters, indicating that metal pollution might affect diet composition and quality. We found strong among brood variation in nestlings for all variables (except cholesterol), suggesting that there might be a considerable genetic and/or parental investment factor involved.