Two field experiments were performed in a Central Spanish population of Blue Tit (Parus caeruleus) breeding in nestboxes in order to elucidate if food availability at laying constrains clutch size (experiment 1), or if clutch size affects nestling growth and survival (experiment 2). In the first, mealworms were offered at the nestboxes of certain pairs during the laying days. No effect of food supplementation on clutch size and egg mass was detected. There was a significant advancement of the onset of incubation in the supplemented group, with the consequent advancement of hatching date. Females used only the extra food to combine laying and incubation earlier. In the second experiment, two chicks were added or removed soon after hatching, and fledging success, fledgling mass and size and starvation mortality compared between the reduced, control and enlarged broods. Chicks in enlarged broods suffered a higher starvation mortality and fledged with lower masses than in control and reduced broods, while there were no effects of the brood manipulation on parental condition. Clutch size in the study population is not constrained by food availability at laying, and affects nestling fitness presumably through the feeding capacity of individual breeding pairs.