Physiological functions of growing nestlings are thought to be traded-off in relation to rearing conditions, with the resulting physiological state of fledglings having important long-lasting consequences for their fitness. By manipulating brood size up and down, and, separately, by supplying additional food (mealworms — larvae of Tenebrio molitor) we tested if alterations of the rearing conditions would influence nestling performance in Blue Tits Cyanistes caeruleus and Great Tits Parus major. Brood size manipulation affected body mass, heterophil-to-lymphocyte ratio (H:L) and fledging probability in both species and the level of triglycerides in nestling Great Tits. Extra food supply influenced only fledging probability, with no other effect on indicators of nestling performance. An effect on nestling body mass and a lack of effect on cell-mediated immune response in the brood-size experiment suggest that nestlings in enlarged broods sacrificed growth to maintain immunity. In general, effects of both types of experiments were probably to some extent masked by specific character of the study site — an urban parkland with high human-induced disturbance.