Theory of Evolution of Nest Parasitism in Birds

In birds, interspecific nest parasitism is generally thought to have evolved via intra-specific nest parasitism. In this article, the evolutionary dynamics of both intra- and interspecific parasitism are analyzed by using a quantitative genetic model. The birds were assumed to be capable of laying their eggs in the nests of either the same or a different species and to evolve to the optimal rates of intra- and interspecific parasitization. Four traits are considered in the model: recognition ability for conspecific eggs, clutch size, total parasitization rate, and interspecific parasitization rate. The model includes five predictions. First, the original form of nest parasitism is likely to be intraspecific. Second, nest parasitism of either type can evolve only when the cost of intraspecific parasitism is smaller than that of competition among offspring in a nest. Third, interspecific nest parasitism should evolve when the costs of both inter- and intraspecific parasitism are small. Fourth, if the marginal decline in survival rate of parasitic eggs in the nests of a different species per unit increase in egg number is greater than the cost of interspecific parasitism, two-species parasitism, in which birds lay eggs in nests of both the same and a different species, should evolve. Finally, if the marginal decline in survival rate is smaller than the cost of interspecific parasitism, complete interspecific nest parasitism should evolve, in which all eggs are laid in the nests of a different species.