It is widely known that parents invest into reproduction to increase offspring survival and thereby increase reproductive success. In particular, female birds allocate maternal resources including lipids, hormones, and nutrients into the egg yolk. Carotenoid pigments, which are responsible for the characteristic yellow, orange, and red colours, are particularly important as antioxidant for the developing embryo. The Investment Hypothesis addresses variable female allocation of maternal resources into egg yolk and predicts that high quality females and females paired to high-quality males will invest more in their eggs. We test predictions of the Investment Hypothesis in the Diamond Firetail Stagonopleura guttata. In captive females and their pair males, we compare yolk carotenoid (lutein and zeaxanthin) allocation and egg size in relation to traits indicative of individual quality (body condition, flank spot number, bill and rump colour). Females with more white flank spots laid larger eggs that contained more lutein, the major carotenoid found in the Diamond Firetail eggs. Total carotenoid concentration in egg yolk was not related to any other female characteristic. Females laid larger eggs when paired with a high quality male (dark red rump).