Haematological health state indices of reproducing Great Tits: a response to brood size manipulation

1. A brood size manipulation experiment was performed in two Great Tit (Par us major L.) populations in order to evaluate the effect of raising differe nt 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 incre ased reproductive effort causes immunosuppression. 3. Haematocrit increased in response to brood enlargement, suggesting a res ponse 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, heterophi le count, intensity of Haemaproteus blood parasite infection or plasma prot eins could be detected. 6. Health state indices were more sensitive to brood size manipulation in t he Great Tits breeding in a rural habitat than urban birds.