Songbird brood sex ratio varies in response to male attractiveness and rearing conditions

Empirical evidence shows that brood sex ratios can be influenced by environmental factors and reproductive success–related parental traits. The Trivers–Willard hypothesis (TWH) predicts condition-dependent adjustment of offspring sex ratios and has been extended to suggest that females mated to more attractive males will bias offspring sex ratios towards sons, as sons that resemble their fathers could yield greater fitness returns through increased grandoffspring production. Although extensive research has explored the effect of male attractiveness on sex allocation, its interplay with environmental conditions is often overlooked. This study examines how brood sex ratios respond to both male attractiveness and annual rearing conditions (ARC; proxied by annual breeding success) within the framework of the TWH. Using long-term data from 2759 molecularly sexed nestlings of pied flycatchers, Ficedula hypoleuca , ringed between 1997 and 2018 (with 643 brood sex ratios determined from eight breeding seasons between 1997 and 2011) and by tracking lifetime fledgling production through the 2023 breeding season, we found support for sex ratio adjustment as predicted by the TWH. Male attractiveness and ARC positively affected the proportion of males in the broods. These associations were partially context-dependent, with each factor becoming influential only when the other was unfavourable. Analyses of nestling mortality before genetic sexing at day 13 after hatching showed that nestling mortality was more frequent in female-biased broods, decreased with favourable rearing conditions and increased with male attractiveness. Importantly, excluding broods with early nestling mortality left the main sex ratio conclusions unchanged. In addition, we did not find statistical evidence for an association between male attractiveness or ARC and offspring’s lifetime reproductive success. Our findings suggest that sex allocation is influenced by poor rearing conditions, sexually selected traits and their interplay. Further research is needed to confirm this relationship and to clarify the fitness consequences for sons and daughters. • Both male attractiveness and rearing conditions influence offspring sex ratios. • Supports Trivers–Willard hypothesis: parity in good conditions, daughter bias in poor. • Male attractiveness and rearing conditions interact in a context-dependent way. • No effect of male attractiveness or rearing conditions on offspring fitness detected. • Long-term data highlight the importance of environmental context for sex allocation.