Theory predicts that parents should adjust reproductive investment in a current breeding attempt by considering the relative fitness benefits of current and future reproductive attempts. Empirical tests, however, have proved problematic because of the difficulties in isolating variables that yield clear and predictable fitness returns to individuals and because partner compensation in socially monogamous species is likely to confound individual investment strategies. We test the effect of parental investment by males and females in the color polymorphic Gouldian finch (Erythrura gouldiae), a species with mutual mate choice and high fitness costs when breeding with incompatible partners. Using a within-individual experimental design, in which both males and females were forced to breed with mates of the same (matched) and different (mixed) color morph, we show that females, but not males, increased their provisioning effort when breeding with compatible mates. By crossfostering offspring within and between matched and mixed pairs, we also found that foster offspring reared by matched pairs, with increased female provisioning, were healthier, grew and developed faster, and fledged earlier than offspring reared by mixed pairs. Furthermore, due to the experimental design, these effects were directly mediated by differential investment by females and not by male compensation. Thus, our results provide support for maternal (but not paternal) effects in response to mate quality.