Sibling Competition and the Evolution of Brood Size and Development Rate in Birds

Parents and their offspring may come into conflict over both the level of food provisioning to the brood and the distribution of food among nestlings within a brood. Sibling competition within a brood favours nestlings whose rapid development increases their competitive status. This may in turn reduce the overall fitness of parents by increasing the food requirements of individual nestlings. Parents can reduce conflict with their offspring by reducing the number of young in each brood and by hatching their eggs asynchronously, so that competitive status is determined by age and not by growth rate. Under these circumstances, benefits of slow development, possibly including a more responsive immune system or delayed senescence, may also result in slower growth and reduced food requirements of nestlings. Future studies of parentoffspring relationships should include evolutionary responses to competition between siblings and parental mechanisms for controlling these responses.