Blood parasites, health, reproductive success, and egg volume in female Great Tits Parus major

The effect of different genera of blood parasites on female Great Tits Parus major was studied in central Sweden. Haemoproteus, Leucocytozoon, Hepatozoon, Plasmodium and Trypanosoma were found, but only the last mentioned had any measurable effect on the birds. Female Great Tits infected with Trypanosoma laid smaller eggs, hatched proportionally fewer eggs, and raised nestlings with shorter tarsi and generally worse body condition, compared to females without trypanosomes. However, females with low body mass and short tarsi laid smaller eggs, independent of parasite status. Low body mass was correlated with low levels of haemoglobin, suggesting that parasitic infection, or some other disease, initially had caused the lower body mass. Haematocrit, another variable reflecting health status, was positively correlated with clutch size. However, neither haemoglobin nor haematocrit was related to occurrence of blood parasites.