Intraspecific nest parasitism of cavity-nesting wood ducks: costs and benefits to hosts and parasites

Intraspecific nest parasitism may have few costs to hosts in species with precocial young and limited parental care. We examined costs and benefits of parasitism to hosts and parasites in wood ducks, Aix sponsa. Hosts laid fewer eggs when parasitized, although host egg laying was not related to the number of parasite eggs in a nest. The proportion of eggs that hatched declined with increasing clutch size; most unhatched eggs were laid by parasites. Hatching success of host eggs was not affected by parasitism. Incubation length also increased with clutch size, which prolonged nest exposure to predators, although simulations predicted that this was offset by dilution of predation risk. However, longer incubation potentially posed energetic costs. For a wide range of parasitism levels, this energetic cost was low compared to the costs of laying a replacement clutch. Therefore, abandonment was favourable only when nests were heavily parasitized early in the laying period, which was consistent with observations. Inclusive fitness benefits were possible for hosts if hosts and parasites were related. Modelling costs and benefits from the parasite's perspective, the cost of lower hatching success was more than offset by the benefits of avoiding incubation. For females that incubated a nest and parasitized another female's nest, allocation of eggs between nests was influenced by the cost of incubating an additional egg, the cost of lower hatching success of parasitically laid eggs, and any dilution of predation risk for parasitically laid eggs. These findings help to explain the high prevalence of intraspecific nest parasitism in precocial birds.