Is bill colouration in wild male Blackbirds (Turdus merula) related to biochemistry parameters and parasitism?

Studies of signalling and sexual selection in birds are increasingly focussing on the regulation of the expression of carotenoid-based ornaments. Brighter coloured ornaments are preferred during mate choice and are thought to signal resistance to parasites, immune capacity and health status. As the colouration of integuments is a dynamic trait, changes in colouration potentially reflect changes in. individual condition. The bill of the male Common Blackbird (or Blackbird, Turdus merula) has become a model for studying the implications of carotenoid-coloured integuments in birds, and Blackbird bill colouration has been found to be related to reproductive ability and immune capacity. However, the relationship between bill colouration and parasites and health in this species remains unclear. We have analysed the association between bill colouration, parasites (blood and intestinal parasites) and health status indicators (standard haematological and plasma biochemistry variables) in free-ranging male Blackbirds during the breeding season. Bill colouration was found to be related to body condition, health status, stress and hydration and nutritional status, but the presence or load of the parasite groups studied was not found to be related to bill colouration. Moreover, parasites showed no clear aggregation patterns. Our results suggest that certain physiological constraints rather than parasite infection are the main cause of variability in the colouration of male Blackbird bills.