PREVALENCE AND INTENSITY OF HAEMATOZOAN INFECTION IN A POPULATION OF GREAT TITS PARUS-MAJOR FROM GOTLAND, SWEDEN

The prevalence and intensity of haematozoan infections were studied in a nest box breeding population of Great Tits on the island of Gotland in the Baltic Sea during 1990 to 1992. Adult birds were captured during the nestling period and blood smears taken. Ten taxa of haematozoa belonging to seven genera were identified. Haemoproteus majoris dominated with an overall prevalence of around 76%. There were year and age class differences in both prevalence and mean intensity of infection. Annual differences were probably due to variation in vector abundance and activity in the previous year. Older birds (greater-than-or-equal-to 2 years old) were more often infected, probably as a result of their longer time of exposure to vectors, but had a lower mean intensity of infection than young (1 year old) birds indicating an acquired immunity to parasites. Our results suggest that the frequency of blood parasites in avian populations may primarily be an effect of the abundance and behaviour of suitable vectors, and that birds may have little scope for behavioural adaptations to avoid being parasitized.