Avian brood reduction and parent-offspring "conflict".

Avian brood reduction is widely viewed as a potential source of parent-offspring conflict. Yet empirical evidence belies the existence of conflict; for example, parents create the initial competitive asymmetries that facilitate brood reduction and, in fratricidal species, rarely intervene to stop sibling aggression. Here I examine parent-offspring conflict over brood reduction using a simple game model. Parents establish an initial brood size and level of effort that optimizes their lifetime reproductive success. Offspring can respond by reducing brood size, but if they do so, parents are free to readjust their level of effort. Brood reduction is an evolutionarily stable strategy (ESS) only when offspring fitness is higher following parental readjustment of effort. Model analysis revealed that both conflict and cooperation over brood size are possible ESS(s). Two parental policies of resource allocation among contemporary siblings were examined: equal, in which food is distributed evenly, and despotic, in which food is distributed according to a sibling dominance hierarchy. Parental fitness was diminished slightly by despotic allocation, but the risk of parent-offspring conflict was also diminished. When conflict did occur, the fitness costs and benefits for parents and offspring, respectively, were modest, particularly for despotic allocation. Where parents add "insurance" offspring to the brood, parents and elder offspring may agree over the elimination of redundant offspring, even though the proximate cause of infanticide is sibling rivalry.