Parental care is one of the crucial factors affecting breeding success in birds. The level of parent investment can differ between males and females. We studied the sex-specific nestling-feeding effort by parents of the red-breasted flycatcher. Our study was carried out in the old-growth oak—lime—hornbeam stands in the Białowieża National Park (NE Poland). The frequency of nestling-feeding and relative nestling-feeding effort (number of visits by male or female expressed a percentage of all visits by both sexes at the nest) depended on sex of parents and age of nestlings but not on their number. Both sexes increased their feeding frequency as the nestlings became older. Sex differences in nestling-feeding have been found in a number of biparental-care bird species. There are many hypotheses to explain this phenomenon. In the red-breasted flycatcher, the most likely reason for differences in feeding efforts are differences in roles the parents play at different stages of brood develompent.