Secondary brood sex ratio in little owls (Athene noctua): patterns and determinants

In bird species with sexual size dimorphism, secondary brood sex ratio is often biased. Two general mechanisms can lead to this pattern. Either adaptive reproductive strategies result in a biased primary sex ratio or parental constraints or restraints result in a sex-biased nestling mortality. Using a big sample of sex determination data of little owl nestlings (Athene noctua) we examined the secondary brood sex ratio, sexual size dimorphism of nestlings, and factors affecting the survival of the owlets during the nestling period. Brood sex ratio in little owl broods was female-biased at fledging but not until the middle of the nestling period and it was female-biased at the end but not at the beginning of the breeding season. We found that female nestlings showed higher body mass than male nestlings of the same rank. A survival analysis corroborated that high-ranked (i.e. late-hatched) nestlings of low body mass showed reduced survival, resulting in a male-biased nestling mortality. We conclude that under limited food input to the nest the brood sex ratio of fledging juveniles is female-biased. This pattern of food-dependent secondary brood sex ratio can have important effects on the demographic parameters of endangered species.