Intraspecific brood parasitism can increase the number of eggs that an individual lays in its own nest

Intraspecific brood parasitism involves laying eggs in the nest of another individual of the same species without subsequently caring for the eggs or hatchlings. Where individuals lay in their own nest as well as parasitically, previous works predicted that parasitism leads to fewer eggs being laid in an individual's own nest, compared with the equivalent situation without parasitism. This is predicted to occur both to reduce the effects of competition from parasitically laid individuals and because parasitism can be used to reduce competition between siblings. Here, we present a quantitative model of this situation. This model indicates that the brood-reduction prediction does not hold universally. For some plausible parameter combinations, parasitism causes an increase rather than a decrease in the number of eggs laid by an individual in its own nest. This occurs because parasitism becomes a less profitable tactic as more eggs are laid by nest owners due to increasing within-nest competition. Increasing clutch size will also increase competition between the host's own offspring, but can still be advantageous if the induced reduction in parasitism is sufficient.