Brood sex ratio in relation to maternal age and timing of breeding in the white-tailed eagle (Haliaeetus albicilla)

Abstract

In populations of wild animals, it may be advantageous to produce one sex of offspring rather than the other. The sex allocation theory suggests that under favorable conditions, parents should bias offspring sex toward the more expensive sex to maximize their lifetime reproductive success. The Trivers-Willard hypothesis more specifically predicts that mothers in better condition should produce the more "expensive" sex. In species with reversed sexual size dimorphism, such as most raptors, females are larger and more energetically demanding to rear than males. A mother’s age and her access to food are likely the biggest factors in deciding whether she produces more males or females. The timing of breeding within a season may also contribute to variation in brood sex ratios. However, most raptor species are hard to detect in the wild, and it takes years for them to reach adulthood. As a result, little is known about the contribution of maternal age, timing of breeding and their interplay in brood sex ratio variation. In this study, I explored whether maternal age and timing of breeding predict brood sex ratio. I predict that older females and earlier breeders would produce more female-biased broods, and that older females breed earlier. For this purpose, I used a unique long-term monitoring data set of a recovering raptor species, the Finnish white-tailed eagle (Haliaeetus albicilla). I used two different datasets to answer my research questions. (1) A large timing of breeding dataset comprising 1243 observations from 390 territories over 21 years (2003 – 2023), and (2) a maternal age dataset comprising 136 observations from 83 individually marked females over 16 breeding seasons (2007 – 2023). The different datasets were used, as maternal age is only known in a subsample of nests. I used generalized linear mixed models with binomial error distributions to test whether brood sex ratios changed within season and with maternal age. Maternal age showed a significant positive association with female offspring production when accounting for non-linear age effects, with older females tending to raise more female-biased broods to ringing age, indicating possibly a plateau effect with increasing age, though the senescence effect cannot be fully ruled out as the sample size is small for older females (n = 3 above age 20). The main finding was primarily driven by young mothers having a high proportion of male offspring (30% female offspring), and older mothers producing sex ratios at roughly equal ratios (42 – 60% female offspring). Timing of breeding showed no significant effect on brood sex ratio (P = 0.263), and maternal age did not significantly predict timing of breeding (P = 0.261). A significant overall male bias was detected at the population-level, with only 45.8% of all nestlings being female. These findings provide partial support for the Trivers-Willard hypothesis, suggesting that individual-level age-structured effects play a more important role than seasonal timing in shaping sex allocation in this species. The results highlight the importance of long-term individual-based monitoring for understanding reproductive strategies in recovering raptor populations.