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.

• Klíčová slova
• Tengmalm's Owl, food fluctuations, hatching order, inter‐annual variation, nestling mortality, seasonal variation, sex ratio, sexually‐size dimorphism,
• Publikační typ
• časopisecké články MeSH

Nest boxes represent a popular tool to support secondary cavity-nesting species. Surprisingly, the benefits and limitations of nest boxes for target species in different environments are poorly understood. We performed a 3-years experimental study in two different Central European forests to evaluate nest box use and breeding performance of boreal owl (Aegolius funereus) - a species well known for its readiness to occupy nest boxes. Based on territorial vocalisation, two boreal owl populations 200 km apart were similarly abundant in their environments. However, only the boreal owl population in young restored Norway (Picea abies) and blue (Picea pungens) spruce-dominated forests on mountain plateaus readily occupied nest boxes with the occupancy reaching 8-15%. Nest boxes lost their supporting function for the boreal owl in mature Scots pine (Pinus sylvestris)-dominated forests in the lowland, where the nest box occupancy reached 0-1%. As a result, the population of boreal owls that used nest boxes in the young restored forests produced 10 times more fledglings than the population inhabiting mature Scots pine forests. We explain the differences by the contrasting availability of natural tree cavities between the two study areas being much higher in mature Scots pine forests. For the first time, this study documents differences in nest box use despite similar food availability and population size of the target species. The study provides the findings-related recommendations for deploying nest boxes for boreal owls and points out a general lack of practical guides.

• MeSH
• borovice lesní * MeSH
• lesy MeSH
• šlechtění rostlin MeSH
• smrk * MeSH
• Stringiformes * MeSH
• zvířata MeSH
• Check Tag
• zvířata MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH