1. A brood size manipulation experiment was performed in two Great Tit (Parus major L.) populations in order to evaluate the effect of raising different numbers of nestlings on parental health state.
2. Brood enlargement resulted in elevated heterophile:lymphocyte ratios and decreased lymphocyte number in the peripheral blood, indicating that increased reproductive effort causes immunosuppression.
3. Haematocrit increased in response to brood enlargement, suggesting a response to the requirement of elevated oxygen-carrying capacity of the blood during increased work load.
4. Parental body mass revealed a tendency to decrease in response to brood enlargement.
5. No effect of brood size manipulation on total leucocyte count, heterophile count, intensity of Haemoproteus blood parasite infection or plasma proteins could be detected.
6. Health state indices were more sensitive to brood size manipulation in the Great Tits breeding in a rural habitat than urban birds.