Recent avian studies suggest that offspring sex ratios can vary with environmental or parental characteristics. One example is that females may adjust brood sex ratio of fledglings in response to the sexual attractiveness of their mate and are expected to produce more sons when mated with older or more ornamented males. We determined offspring sex ratios at fledging (158 nestlings from 33 broods) in a population of migratory bird species, Red-breasted Flycatchers (Ficedula parva) that were breeding on the western edge of their geographic range, in the strictly protected part of the Białowieża Forest (north-eastern Poland). The ratio of individual broods ranged from all male to all female, but the average offspring sex ratio at the population level did not deviate significantly from the expected 50:50 sex ratio. Moreover, we found no evidence that sex ratio varied significantly with year, with the incidence of extra-pair paternity, with partial brood losses, or with brood size. However, females mated to older males (older males are also more ornamented) were significantly more likely to produce male-biased broods. Our data suggest that female Red-breasted Flycatchers manipulate sex ratio in response to their mate’s age.
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Vol. 127 • No. 2