Extensive research has been conducted to explore adaptive variation in offspring sex ratios, focusing on birds as a model group. However, studies to date have not been comprehensive in scope, limiting our understanding of whether there is substantial within- or among-year variation in offspring sex ratios, which environmental conditions and mechanisms are associated with this variation, and when during a nesting attempt the fledgling sex ratio is largely determined. To address these gaps, we analyzed our 18-year dataset from 542 sexually size-dimorphic Boreal Owl (Aegolius funereus) offspring in 140 nests. At hatching, within-nest variation in hatching order emerged as the primary predictor of sex ratio, with later-hatched offspring more likely to be female in larger broods when one or more eggs failed to hatch; such broods were primarily produced in years of abundant food. No evidence of direct sex-dependent mortality among nestlings was observed between hatching and fledging; instead, sex-independent mortality of nestlings increased for offspring that hatched later in a brood and during years of low Apodemus and Microtus prey abundance. At fledging, the primary predictor of offspring sex ratio was year-to-year variation in food abundance, with more male fledglings produced in years of abundant food and larger broods, but only in nests where one or more nestlings had died. We found no compelling evidence for within-season variation in offspring sex ratio between early- and late-season nests. Our findings suggest that offspring sex ratios in raptors are shaped by a complex interplay of maternal adjustments and environmental influences, particularly food abundance, which drives changes in brood size. These findings emphasize the need for future research to conduct a more comprehensive examination into offspring sex adjustments, particularly focusing on alterations in sex ratio during multiple nesting stages and their association with variation in offspring mortality and environmental conditions.

Cornell Lab of Ornithology New York New York USA

Department of Ecology Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic

Institute of Experimental Botany Academy of Sciences of the Czech Republic Prague Czech Republic

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