Young, immature-plumaged males of species in which plumage maturation is delayed are often unable to obtain mates or pair later in the season to lower-quality females. Their clutches are smaller and the quality of their eggs is lower, leading to fewer nestlings, whose quality and fledging success are reduced. The Saffron Finch (Sicalis flaveola) is an understudied and declining secondary-cavity nester with delayed plumage maturation; yearling males look like females. The aims of this study, of birds nesting in boxes, were to compare the morphology of immature- and mature-plumaged males of the Saffron Finch and their respective females and to compare the breeding success of females mated to each category of male. We expected that females mated to immature-plumaged males would follow the pattern described for other species, having lower reproductive investment and success. Immature- and mature-plumaged males were similar in morphology, but females paired to mature-plumaged males were bigger in bill height and weight. We found no differences, however, between females paired to each category of male in any biologically meaningful variable. Reduced competition among males for nest cavities and enough females willing to pair with immature-plumaged males might be partially responsible for these results.