RETARDATION OF BREEDING ONSET IN GREAT TITS (PARUS-MAJOR) BY BLOOD PARASITES

1. Timing of breeding is one of the most important factors for reproductive success in birds and the nutritional state of the female seems to affect breeding time directly. 2. Protozoan blood parasites consume a variety of host metabolites as well as haemoglobin. If blood parasites affect the nutritional state of females, parasite-free individuals will breed earlier than parasitized individuals. 3. Effect of blood parasites on breeding time of Great Tits (Parus major L.) was studied for a population of tits nesting in boxes on the island of Gotland, Sweden. 4. There was a significant negative relationship between infection of blood parasites and timing of breeding in females but not in males. Females with blood parasites initiated egg-laying and hatched their eggs later than uninfected females, after controlling for effects of female age on these variables. 5. We suggest that parasites may compete for energy with the host resulting in a retarded initiation of laying. 6. Because laying date and hatching date have been demonstrated to affect juvenile survival, and because parasites in the present study have been shown to affect laying and clutch hatching date, parasites consequently may reduce individual fitness of birds in this population of Great Tits.