@article{10272/28076,
year = {2026},
url = {https://hdl.handle.net/10272/28076},
abstract = {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 conditiondependent 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.},
organization = {C.C. received financial support from the Spanish Ministry of Science, Innovation and Universities (FJC2018-038412-I), MCIN/AEI/10.13039/501100011033 and the European Union NextGenerationEU/PRTR (RYC2021-033977-I). DC received support through a Talent Attraction fellowship from the Autonomous Community of Madrid (CAM), Spain (2022-T1_AMB-24025) and the projects PID2022-141763NA-I00 and CNS2024-154721 funded by MCIN/AEI/ doi: https://doi.org/10.13039/501100011033. J.M.-P. is supported by research projects granted by the Spanish Ministry of Science and Innovation (PID2019-104835GB-I00 and PID2022-138133NB-I00). J.P. was funded by project CGL2015-70639 of the Spanish Ministry of Economy and Competitiveness. S.S. was supported by Biodiversa+, the European Biodiversity Partnership, in the context of the WildINTEL project under the 2022–2023 BiodivMon joint call.},
publisher = {Elsevier},
title = {Songbird brood sex ratio varies in response to male attractiveness and rearing conditions},
doi = {10.1016/j.anbehav.2026.123508},
author = {Santoro, Simone and Losada, María and Canal, David and Camacho, Carlos and Martínez Padilla, Jesús and Sánchez Tójar, Alfredo and Potti, Jaime},
}