Plumage brightness in relation to haematozoan infections in the greenfinch Carduelis chloris: Bright males are a good bet

Variation in plumage brightness (yellowness) and prevalence and intensity of haematozoan infections in greenfinches Carduelis chloris L. were studied in three populations, widely separated in geographic location from Spain to Finland. Sexual dichromatism (SD) in coloration was marked, males being yellower than females. Although older birds were generally yellower than younger birds, the degree of SD in coloration increased with age, perhaps due to delayed plumage maturation in males. The prevalence of haematozoan infections was unrelated to any measure of male plumage coloration, but the intensity of haematozoan infections was strongly negatively related to male plumage brightness, and this pattern was similar in all three populations. Hence, male plumage brightness reveals information about the extent to which males are parasitized. This result is in accordance with the Hamilton-Zuk hypothesis, which suggests that bright male plumage has evolved to indicate heritable resistance to parasites. The fact that lightly-infected males had brighter plumage than non-infected and heavily infected ones may suggest that non-infected males were a mixture of both susceptible and resistant individuals, whereas lightly infected individuals might have been effectively immune to parasites. Our data provide a clear example of the negative association between plumage brightness and blood parasite loads in birds, and suggest that male plumage yellowness in the greenfinch can function as an indicator of male quality.