(1) Brood reductions and enlargements were carried out in kestrel nests to evaluate the consequences of raising different numbers of nestlings for both the offspring and the parents. (2) Brood enlargements caused increased daily hunting activity of the parents, reduced growth rate of the nestlings, increased nestling mortality and enhanced weight loss in the female parent. Brood reductions caused an increased food intake by the nestlings, in spite of (non-significantly) reduced parental hunting activity. Local survival of the parents was negatively correlated with the experimental change in brood size. (3) A review of the literature on brood enlargements is presented, showing that parents were able to raise more young till fledging than their natural broods in twenty-nine out of forty altricial bird species investigated. Negative effects of brood enlargements on parental survival or future reproduction were established in eight out of twelve species investigated. (4) The results are consistent with the theory that parental work for the offspring entails an inherent reduction in future reproductive output and that natural broods, by being smaller than the maximum number of nestlings that can be raised, maximize the total reproductive output.