Abstract: Although biased offspring sex ratios are common in species with sexual size dimorphism, the proximate causes are often unresolved. This is because two general mechanisms operating in different ways and in various periods of reproduction can lead to the bias: sex-biased survival or parental sex-allocation. We examined nestling sex ratio patterns between hatching and fledging, sexual size dimorphism, and factors affecting nestling survival using growth and survival data of 846 individual little owl Athene noctua nestlings with known sex from 307 broods from Germany, the Netherlands and Denmark. Nestling sex ratio was female-biased, mainly due to a significant female bias in the first-hatched chicks. Females showed a higher body weight than male nestlings at ringing and body weight of nestlings decreased with hatching sequence. Nestling survival was higher in females (Φ = 0.91) than in males (Φ = 0.85), and survival rates were positively related to body mass and negatively to brood size. Although the observed lower survival of males can cause an overall female-biased sex ratio, the sex dimorphism and survival patterns found here are unlikely to explain the conspicuous sex ratio pattern with a female bias in the first-hatched nestlings and the increase in female bias across the season. Thus, these results point towards interacting mechanisms of parental sex allocation strategies and sex-specific survival. As the female bias was allocated to the first rank that is most likely to survive, the female bias will increase under suboptimal breeding conditions. We therefore suggest that under suboptimal ecological conditions, higher investment into females is adaptive in little owls. Significance statement: Biased sex ratios can have severe effects on the social behaviour and population dynamics of endangered species. However, the existence of subtle sex ratio bias is often unknown and its proximate mechanisms and ultimate consequences often remain unclear. Small sample sizes make the detection of subtle effects unlikely and often fail to disentangle diverging mechanisms such as sex-biased survival and parental sex allocation. We used a large dataset of 846 little owl nestlings from 307 broods from three countries to investigate offspring sex ratio patterns, sexual size dimorphism and nestling survival simultaneously. Our findings suggest interacting mechanisms of parental sex allocation strategies and sex-specific survival to drive biased offspring sex ratios in little owls. The context dependence of the sex ratio bias indicates that offspring sex ratio bias in little owls is both, a consequence of—and an adaptation to—suboptimal breeding conditions.