Sex-biased survival of nestlings under unfavourable conditions drives secondary sex ratio in little owls (Athene noctua)

Abstract

Identifying variation in key life history traits is crucial to predict population dynamics of endangered species. An increased bias in sex ratio as a result of low habitat quality can reinforce population decline and drive small populations to extinction. This is particularly true for monogamous bird species such as the little owl (Athene noctua). However, the underlying mechanisms driving biased sex ratios often remain elusive. Using a robust sample of molecular sex determination data recorded at two nest visits within each of 247 broods we examined the development in brood sex ratio, sexual size dimorphism and factors affecting the survival of little owl nestlings. Brood sex ratio in little owl broods was female-biased at fledging but not before the middle of the nestling period and it was female-biased at the end but not at the beginning of the breeding season. Female nestlings showed higher body mass than male nestlings of the same hatching rank. A survival analysis corroborated that late-hatched nestlings of low body mass showed reduced survival under adverse habitat conditions, resulting in a male-biased nestling mortality. Our conclusions therefore support theoretical predictions according to which the more sensitive sex will be disproportionally affected by unfavourable conditions that were previously identified by feeding experiments in the study system. This pattern of condition-dependent secondary brood sex ratio can severely affect the demographic parameters of endangered species and provides important implications for conservation.