Nest boxes for breeding tawny owls Strix aluco were sited in a mixed oak/hornbeam/beech forest located in the Pilis Biosphere Reserve, 30 km northwest of Budapest, Hungary, during the period 1992–2000. We hypothesized that both within-pair differences in age composition and weather conditions affect the breeding success of the parents. To test our hypothesis, we marked the owl parents in their first known breeding year and related their reproductive performance was to within-pair age composition through three subsequent breeding seasons. Sex-related age differences affected the breeding performance of the parents in their first and second known breeding year: number of eggs and hatching success were influenced by the age of females, while fledging success was influenced by the age of males. Within-pair age differences did not affect the breeding performance at their third known breeding year. Reproductive performance was lower in snowy years than in years without snow cover in all of their three subsequent breeding seasons. On basis of the lower mean body mass and shorter mean tarsi of the nestlings before fledging recorded in adverse weather conditions, we suggest that parents favour the sex that demands less investment in care. As a consequence, the sex-ratio becomes skewed towards the male in a poor food environment.