Increased variance in the reproductive success of males relative to females favors mothers that optimally allocate sons and daughters to maximize their fitness return. In altricial songbirds, one influence on the fitness prospects of offspring arises through the order in which nestlings hatch from their eggs, which affects individual mass and size before nest leaving. In house wrens (Troglodytes aedon), the influence of hatching order depends on the degree of hatching synchrony, with greater variation in nestling mass and size within broods hatching asynchronously than in those hatching synchronously. Early-hatching nestlings in asynchronous broods were heavier and larger than their later-hatching siblings and nestlings in synchronous broods. The effect of hatching order was also sex specific, as the mass of males in asynchronous broods was more strongly influenced by hatching order than the mass of females, with increased variation in the mass of males relative to that of females. As predicted, mothers hatching their eggs asynchronously biased first-laid, first-hatching eggs toward sons and late-laid, late-hatching eggs toward daughters, whereas females hatching their eggs synchronously distributed the sexes randomly among the eggs of their clutch. We conclude that females allocate the sex of their offspring among the eggs of their clutch in a manner that maximizes their own fitness.