Age and sex are key information to interpret results of studies on birds' biology. As sexing monomorphic species is difficult, in this study we aimed to determine the degree of sexual size dimorphism in song thrushes migrating across the southern Baltic coast, and to develop reliable sexing criteria for future studies of this species. We ringed, measured and took blood samples of song thrushes at the Polish coast. Then we used molecular techniques to determine the sex of ringed individuals and to calculate functions from morphometrics for sexing birds from different age groups. Our results showed a degree of size dimorphism between sexes, more pronounced in adults than in immatures. Dimorphism was clearest in the size of the vestigial outermost primary, which was longer in females despite their smaller size, a likely result of different selection pressures during the breeding season. Differences between sexes were greater in adults than immatures, probably because the immatures are not exposed to selection pressures acting on older birds during migration and breeding. We developed equations allowing to estimate sex correctly of 81.5% of adults in spring and 81% in autumn, and of 63.3% immatures in spring and 63.6% in autumn. Thus, the equation for adults can be used with confidence, while those for immatures with caution.