Female reproductive investment in a conspecific brood parasite

Conspecific brood parasitism occurs when a female lays an egg in another female's nest of the same species and subsequent care is provided by the host. Parasitism may have a substantial influence on how both parasitic and host females adjust their level of reproductive investment. I investigated patterns of female reproductive investment in the wood duck (Aix sponsa ), focusing on nest site choice, parentage, egg investment, and incubation behavior. Females must choose a nest site in which to lay, regardless of strategy, yet the cues they use are poorly understood. I found that females use the number of eggs in a nest to evaluate a nest. Kinship and social relationships do not influence nest site choice. DNA analysis of maternity revealed the surprising result that some nests did not contain any eggs of the host female. Females also invest in egg production, and the amount of nutrients (i.e. yolk androgens) provided to the offspring may differ between females and reproductive strategies. Yolk androgens have been shown to influence embryonic development with higher concentrations increasing developmental rates. Parasitic eggs had higher androgen concentrations, possibly facilitating synchronization with the host's eggs and allowing late-laid eggs to have higher hatch success than expected. Host eggs contained higher androgen concentrations in later laid eggs, which may facilitate hatching synchrony of their own eggs. Finally, host females may differ in their investment in incubation behavior and in their consistency in incubation. Females exhibited significant differences in incubation length and temperature, but were also influenced by the environment and brood parasitism. Individuals were consistent across seasons only in incubation constancy. Incubation constancy and temperature had a significant positive influence on duckling size at hatch. The four components of my research (nest site choice, kinship, egg investment, and incubation investment) reveal the rich complexity in how females might adjust their level of maternal investment in the face of varying environmental and social conditions (i.e. brood parasitism). I suggest that, because of the wide range of reproductive investment possible, brood parasitic species provide a model system to explore further both proximate and ultimate functions of this complexity.