Host life-history strategies and the evolution of chick-killing by brood parasitic offspring

Nestling brood parasites vary in the harm that they do to their companions in the nest. Here we use a game-theoretical model to attempt to account for this variation. Our model considers hosts that might routinely abandon single nestlings, regardless of whether they are host young or brood parasites and choose instead to reallocate their reproductive effort to future breeding. The nestling brood parasite must decide whether or not to kill all host young by balancing the benefits it stands to gain from reduced competition in the nest against the risk of desertion by host parents. The model predicts that 3 different types of evolutionarily stable strategies can exist. 1) When hosts routinely rear depleted broods, the brood parasite always kills host young, and the host never then abandons the nest. 2) Conversely, when adult survival after deserting single offspring is very high, hosts always abandon broods of one young, and the parasite never kills host offspring. 3) Intermediate strategies can also be evolutionarily stable, in which parasites sometimes kill their nest mates, and host parents sometimes desert nests that contain only a single chick. We provide quantitative descriptions of how the values given to ecological and behavioral parameters of the host-parasite system influence the probability of each strategy and compare our results with host--brood parasite associations seen in nature. Copyright 2008, Oxford University Press.