For decades, variation in offspring sex ratios in birds has been a key topic in animal demography and population dynamics. In raptors, different factors such as food availability and breeding phenology may significantly influence sex ratios, while the impact of climatic conditions remains poorly documented. Here, we explored individual and environmental factors potentially affecting the secondary sex ratio of the long-lived, single-egg layer cinereous vulture Aegypius monachus . Using data collected over a 20-year period (2004–2023) in two breeding areas of central Spain, we assessed the effects of food availability, breeding phenology, climatic conditions, and spatiotemporal variability on offspring sex ratio ( n = 186 nestlings). Offspring sex ratio was not biased either at the population, area or nucleus level, but deviations from a paired sex ratio were found in specific years and nuclei. Offspring sex ratio was not influenced by a food availability proxy (a two-level factor reflecting changes in livestock carcass availability derived from sanitary regulations implemented after the mad-cow crisis), and showed a tendency to be affected by breeding phenology, with more male offspring at the end of the breeding season in the high-altitude area. Moreover, temperature in the months prior to fertilization, but not after, affected nestling sex. Both cold and warm winters resulted in a predominance of male nestlings, while intermediate temperatures were associated with more females. Although the underlying mechanisms remain unclear, the temperature-associated sex ratio bias may be due to female sex manipulation or as an epiphenomenon associated with hormonal-mediated sex determination. These results emphasize the importance of studying how pre-fertilization climatic conditions might affect sex determination, and suggest that deviations in offspring sex ratios in long-lived species may be shaped by complex interactions between individual and environmental factors.
Gómez‐López et al. (Mon,) studied this question.