When the overall food demand of the young increases, parents can either increase their effort to feed the brood, potentially reducing their residual reproductive value, or alternatively maintain their effort, leading to offspring mortality. In long-lived species where fitness is related to the number of breeding attempts, life-history theory suggests that parents should restrict any increase of reproductive effort in a current brood so as not to compromise their survival prospects. We investigated this hypothesis in the Barn Owl Tyto alba by performing brood size manipulations. We enlarged or reduced broods by two nestlings to create some broods requiring more parental investment and others requiring less. We monitored the effect on the parents and the offspring. Total body mass gained by all nestmates from the 24th to the 25th day after the first hatching, a measure correlated with parental feeding rate, was not significantly different between enlarged and reduced broods. Body mass and body condition of male and female parents during the manipulation, renesting rate and their reproductive success measured the year after the manipulation were not significantly affected by the experiment. Nestling mortality was higher, and body mass of the surviving male and female nestlings was lower in enlarged than reduced roods. In conclusion, we detected an effect of brood size manipulations on nestlings but not on parents. In the Barn Owl, this suggests that when broods require extra parental effort, parents do not jeopardize their future reproductive success, and brood reduction occurs.