The evolution of brood reduction by siblicide in birds

The young of a number of bird species frequently kill their siblings whilst still in the nest. We develop simple single-locus genetic models of the evolution of siblicide and non-siblicide in bird populations. Two cases are distinguished: (1) where there is an obvious runt which, when siblicide occurs, always perishes and (2) where a siblicidal individual chooses a victim at random from its nest mates. The genetic models are compared with an inclusive fitness analysis of siblicide due to O'Connor (1978, Anim. Behav. 26, 79–96). In the runt model, the genetic analysis and inclusive fitness analysis yield the same results though the genetic analysis reveals a large area of parameter space where both strategies are evolutionarily stable strategies (ESSs) but only one is a continuously stable strategy (CSS). In the model without a runt, the two analyses give different results and for a large area of parameter space both siblicidal and non-siblicidal behaviours are stable to invasion (both ESSs and CSSs) so that the observed state of a population will depend on its past history.