Brood reduction in birds: Selection for fratricide, infanticide and suicide?

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Kin selection and inclusive fitness concepts are used to formulate a general theory accounting for the phenomenon of brood reduction in birds. The theory shows that as starvation mortality increases, selection favours at first fratricide, then fratricide plus infanticide, and finally fratricide, infanticide and suicide (by the nestling with the shortest life expectancy). A theoretical treatment of the alternative option, food sharing accompanied by concomitant reduction in growth rate, shows the latter to be ineffective when nest predation is high. Brood reduction is not limited in this manner. The theory allows several testable predictions about the incidence of starvation, starvation mortality, and sibling aggression, as well as their brood-size dependence and timing. These predictions are largely supported by the available evidence.