The fitness–related consequences of egg mass, independent of confounding influences associated with parental quality, remain poorly understood for wild birds in general and for passerines in particular. We performed cross–fostering experiments to test the hypothesis that egg mass, independent of parental quality, is the primary determinant of fitness–related traits in nestling house wrens (Troglodytes aedon), an insectivorous passerine. Nestling mass was significantly correlated with the mass of the eggs from which nestlings hatched early but not late in the nestling period in early–season broods. In contrast, in late–season broods, nestling mass was correlated with egg mass until nestlings achieved asymptotic mass. Neither nestling growth nor survival to nest leaving was related to egg mass in either early– or late–season broods; however, nestlings in late–season broods grew more slowly than did nestlings in early–season broods. We propose that nestling mass and egg mass remained correlated throughout the nestling period in late–season broods because decreased arthropod food resources late in the breeding season constrain parents' ability to provision nestlings. We conclude that female house wrens in this population trade–off clutch size for greater egg mass to maximise reproductive success in late–season broods.