INTRASPECIFIC VARIATION IN PLUMAGE COLORATION REFLECTS IMMUNE-RESPONSE IN GREAT TIT (PARUS-MAJOR) MALES

1. The Hamilton and Zuk hypothesis proposes that within species, individuals with more elaborate secondary sexual traits signal their higher resistance to parasites. 2. Intraspecific tests of the correlation between secondary sexual traits and parasite load have given equivocal results, probably because of difficulties in deciding which parasite species are important for the process of sexual selection. 3. In this study we use a different approach by relating secondary sexual traits to the level of immune response in the hosts rather than the parasite load. It is well known that the number and proportions of different types of leucocytes reflect the health status of individuals, as these cells quickly respond to stress and infectious diseases. 4. The number and proportions of different types of leucocytes were counted in blood smears from male Great Tits, Parus major and the intensities of the males' yellow plumage coloration were scored, in 1991 and 1992 on Gotland, SE Sweden. 5. We found a positive correlation between number and proportion of heterophils and colour intensity. 6. More than one interpretation is possible though we favour the view that brighter males signal low parasitation intensity as a result of their superior immunity.