Our results suggest that female wood ducks can distinguish between nests containing different numbers of eggs and use this information to guide their decision to lay in the nest and, potentially, to incubate the combined clutch. The ability to count has been found in a variety of taxa and at least one other brood parasitic species (American coots) have been reported to count eggs to manipulate clutch sizes (Lyon 2003 Nature 422: 495-499). By assessing the number of eggs in a host nest, a potential brood parasite may be able to better evaluate the laying stage and/or prospective value of a nest. For some females, nests with small number of eggs were sufficiently valuable that females incubated the clutch, despite the fact that other eggs were already present.
Furthermore, hosts showed higher androgen concentrations in later laid eggs, which may assist in synchronizing the development and hatch of their own eggs. Parasitic eggs had higher androgen concentrations than host eggs, and eggs laid after the initiation of incubation had higher hatch success than expected, suggesting a possible role for androgens in hatch synchrony. Our study suggests yolk androgens may play a role in hatch synchronization in precocial birds and a role in assisting conspecific brood parasites in parasitizing the host female. While host females may utilize androgens to influence hatch synchrony, conspecific brood parasites may coopt this ability to influence offspring development and increase the success of their eggs and exploit their host.