Effect of brood size manipulations on parents and offspring in the barn owl Tyto alba

When the overall food demand of the young increases, parents can either inc rease their effort to feed the brood, potentially reducing their residual r eproductive value, or alternatively maintain their effort, leading to offsp ring mortality. In long-lived species where fitness is related to the numbe r of breeding attempts, life-history theory suggests that parents should re strict 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 parent al investment and others requiring less. We monitored the effect on the par ents and the offspring. Total body mass gained by all nestmates from the 24 th to the 25th day after the first hatching, a measure correlated with pare ntal feeding rate, was not significantly different between enlarged and red uced broods. Body mass and body condition of male and female parents during the manipulation, renesting rate and their reproductive success measured t he year after the manipulation were not significantly affected by the exper iment. Nestling mortality was higher, and body mass of the surviving male a nd 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 par ental effort, parents do not jeopardize their future reproductive success, and brood reduction occurs.